Sorry Little_Boot, I cannot be of help due to no experencie with that…
I think it will indeed unbalance the game when simply doubling all pieces by type in an area. However, it seems to me you can double just the number of pieces in each area (not necessarily and in some cases definately not by type!), but you will still need to test each chance for gamebalance.
For example, if you double Japan’s pieces, instead of doubling its aircraft, I’d place an extra INF in each area for each present plane. Or CA/BB in the case of a SZ. Same story for Germany (ARM, ART or INF instead of AIR). For the allies a completely diffent approach would be needed. Creative thinking for each Major Power apart. If you keep their historical strengths and weaknesses in mind, it could work. But imho you can’t escape some serious game tesing. My two cents of thinking.
Good luck with your project!