i'm not sure that this would be a good idea. I see two options to achieve that goal:
1) recreating the game simplifying the models, the maps, ecc...
it may work, but will result in having 2 games instead of one to maintain and a splitted userbase.
2) acting on ioq3 to lower requirements maintaining the game contents as they are.
i don't see that one as a viable option for ioq3.
and, anyway, a 300 and more mb are a lot if someone has a 20/10 GB or even less hd...
1) for models there would be both versions of the model in the same .blend file.
seperate lo-fi map brush geometries wouldn't be a good idea.
2) this game is not ioq3 based nor is it even q3 based. i'm also aiming for small filesize (20mbish precompressed)