As promised, here’s a summary of the information I was able to find about the zero-IPC islands on the Pacific 1940 map. The sources I consulted weren’t as detailed as I’d hoped, so the summaries below are basically an estimate of what the general situation was for each island group. Some of these assessments could be off the mark, so if anyone has access to better data please feel free to correct whatever errors exist or to provide supplementary information.
Aleutian Islands
Important naval base(s)? No, just a US naval station (pre-war and onward) at Dutch Harbor whose facilities were very limited.
Important air base(s)? To some degree. Several US forward airbases were established (pre-war and onward), with larger bases like Elmendorf in Alaska proper. The Aleutians have very poor flying weather.
Caroline Islands
Important naval base(s)? Yes. Truk was a major Japanese naval base, pre-war and onward. The Japanese did not make significant use of Ulithi Atoll, but after the American capture of the Carolines the US developed Ulithi into a major forward naval base that was used in the last year of WWII.
Important air base(s)? Yes, as an adjunct to the naval bases.
Dutch New Guinea
Important naval base(s)? To some degree. Hollandia had a good anchorage which both the Japanese and (from 1944 onward) the US used to some extent, but its facilities were minimal.
Important air base(s)? To some degree. Various airfields were built by the Japanese after their occupation began, and were used by the US from 1944 onward. There was an airstrip at Hollandia.
Fiji
Important naval base(s)? No, but there was a good harbour at Suva.
Important air base(s)? No, just a UK wartime airstrip built after 1941.
Note: The Ellice Islands, geographically located about halfway between Fiji and the Gilbert Islands but not appearing on the Pacific 1940 map, were used as naval and air bases by the US in WWII. In particular, there seems to have been an important airbase on Funafuti Atoll.
Gilbert Islands
Important naval base(s)? No. Port facilities at Tarawa were almost nonexistent.
Important air base(s)? To some degree. An airstrip was built by the Japanese on Tarawa in 1942; it was taken over by the US in late 1943. The nearby Phoenix Islands, under joint US/UK control, had a number of airstrips established on them during WWII, for instance on Enderbury Island and Canton Island.
Guam
Important naval base(s)? No. At the beginning of WWII it only had a small harbour with few facilities, though there were some good anchorages here and there, notably at Apra. Captured by Japan in December 1941; recaptured by the US in August 1944.
Important air base(s)? To some degree.
Johnston Island
Important naval base(s)? No, served only as US sub refueling base. It had no port and no decent anchorge.
Important air base(s)? Yes, US, existed pre-war and grew in wartime.
Line Islands
Important naval base(s)? No.
Important air base(s)? To some degree. There was a US naval air station on Palmyra Atoll, pre-war and onward. Palmyra was well positioned to control part of the airspace lying directly on the U.S.-to-Australia route. Nearby Christmas Island also had some US airfields.
Marianas
Important naval base(s)? No. Tinian lacked a proper port.
Important air base(s)? Yes. It had a Japanese pre-war base. Tinian was captured by the US in mid-1944 and was developed by them into a massive airbase. Fleets of B-29s operating from Tinian bombed Japan extensively; the two A-bomb missions took off from there.
Marshall Islands
Important naval base(s)? Yes. Japanese naval and air bases of various sizes were established there in the late 1930s, notably at Kwajalein, Majuro, Eniwetok and Bikini, but the Japanese did not develop them to the extent that they could have. After the capture of the Marshalls by the Americans in January 1944, the US Navy used Majuro as a major forward naval base.
Important air base(s)? Yes.
Midway
Important naval base(s)? No, except as a US submarine base.
Important air base(s)? Yes, US, existed pre-war and grew in wartime.
New Britain
Important naval base(s)? Yes. Rabaul, which has an excellent harbour, was the largest Japanese base in New Guinea during the 1942-1945 occupation.
Important air base(s)? Yes.
New Guinea
Important naval base(s)? To some degree. Allied-controlled Port Moresby apparently had limited use as a port, but the town itself and the bases around it were important Allied staging areas.
Important air base(s)? Yes, several, established both by Japan and the Allies (who controlled different parts of the island) during the war.
New Hebrides
Important naval base(s)? To some degree. Espiritu Santo only had a small port and a small airfield, but it saw a lot of use by the Allies during the war. Nearby Free French-controlled New Caledonia (which is not part of the New Hebrides, but is located nearby to the southwest) was an important Allied forward base in the early stages of the war; it had a small pre-war French naval and air base, good anchorages in the area and a small port at Noumea. The Santa Cruz Islands, northwest of Espiritu Santo, were not used by the Allies despite their advantageous position because the local strain of malaria was too virulent.
Important air base(s)? To some degree.
Palau Island
Important naval base(s)? No. Peleliu had no port facilities.
Important air base(s)? No. There was a Japanese airfield on Peleliu, though apparently not an extensive one. The US captured Palau in late 1944, but made little subsequent use of it to support its operations in the western Pacific.
Samoa (American Samoa and New Zealand Samoa Mandate)
Important naval base(s)? No, just a minor US naval station that existed (pre-war and onward) at Tutuila; harbour traffic increased for the first half of the war but decreased thereafter. The principal port of American Samoa, Apia, only had limited facilities.
Important air base(s)? Yes. US Tafuna Airfield (in American Samoa) existed pre-war and grew in wartime. The US built Faleolo Airfield (in New Zealand Samoa Mandate) in 1942; it was used by US. There was a decent airfield at Apia.
Solomon Islands
Important naval base(s)? Yes. Little or nothing exietd pre-war, but the US and Japan both established several naval bases in the area from 1942 onward.
Important air base(s)? Yes. Little or nothing existed pre-war, but the US and Japan both established a large number of airfields in the area from 1942 onward, Henderson Field on Guadalcanal being perhaps the most famous one.
Wake Island
Important naval base(s)? No. It had no anchorage.
Important air base(s)? No, just a airfield useful for reconnaissance planes and Marine garrison aircraft. Captured by the Japanese in 1941; surrendered in September 1945.