The Phillipines defenately fit into this theatre. Some of the first ground battles were in the Phillipines, (think MacArthur and "I shall Return"). Then you have some great navel and ground battles in 1944 & 45.
Fiji never saw any action, but did serve as a supply base and had a recon unit in 1942 if I remember. Borneo was part of the New Guinie champaign, but you will need to research to determine details. Timor, I am not familar with, but a little research would determine its history.
By the way, to all out there, I just got a book that covers each country's involvement in WWII. There is some fun facts that I didn't know about some of the smaller countries and their role in the war. Anybody need some info. on a country, just send me an email.
The Monogram 1/48 P-38 kit does give some direction to the "M" model. Unfortunately, it does not give any details on the units that used them. The links that were posted earlier by Panzerwulff were very helpful, except I did not find any listings of units that used the plane. I will keep looking, I don't want to give up yet....