Just a thought... the idea is good, but FLIR would be damn hard to implement as a modification.
On way I though about doing for Infiltration (UT Mod) was to replace all, and I mean all, textures. After thinking about it for 2 seconds, I concluded that it's not possible.
If you had low level access to the engine, I would add to everything in the world a temperature and generate the FLIR image with that. But that's how I would do it on the level of the engine or on an engine that allows this kinds of heavy modification. I know it would be possible with UT to start with, but I have my doupts about BF2. Plus it would represent quite a lot of work. Cool thing is that you could easily use dynamic values and see vehicles and your rifle heat up as they are used.
Even if you get to generate a BW image, you'll never have the same effect you have with a FLIR.
--- slightly off topic ---
Speaking of FLIR, that image looks like from a pretty old generation FLIR system. The dynamic range seems pretty low to me. Are those the current systems used on the US attack helicopters or have they (or will they) get into an upgrade phase?
Then again, it could be designed to with a compressed intensity curve so the scene and object scene temperature are dark and anything just a bit hotter stands out more. Could make sens.
Disclaimer: As he was educated in the "Infiltration school of modding", be aware that this user is a realism freak. He cannot be held responsible for any emotional damage this might generate, although he apologizes for it in advance.
They got the newer FLIR systems in service now that would be good on really foggy maps and night maps, past that it wouldnt be a good add.... could see just as good if not better with regular vision than with the FLIR