You may want to post about this in the
DoomWorld demos forum as well, if you haven't done so already. :)
Regarding AVI making, there's at least one hack of prboom floating around that allows this (made by Dashiva), but I don't think it's been released to the general public, and the version I got is rather buggy, too. Still, Peroxyd (aka VinceDSS) has made a couple of AVIs (of regular speedruns, not TAS runs) with it that are available on
my tracker. Note that these are recorded with motion blur - I wouldn't be surprised if this increased the size of the AVIs.
That being said, most of the DOOM speedrunning community seems to be focused more on regular speedruns rather than TAS runs, but unlike in other circles, the two groups are on very friendly terms. In fact, saying "the two groups" is actually misleading, since for the most part, they're one and the same.