I'm a member of a student-run telemedia organization at my high
school, and we're looking for a way to do "real" transparent graphics
overlays for live broadcasts. By which I mean graphics with a full 256
levels of transparency (currently we just have one level through
chroma-keying).

Our video switcher has a DSK function, with inputs for the fill layer
and the key layer. We design all our graphics in Photoshop. So my
question is this: Is there any way, either in vanilla Photoshop or
through a plugin, for us to take an ordinary PSD file with
transparency and save it as a pair of bitmaps, one for the fill and
one for the key?

Bonus points for a solution that also works with After Effects, in
case we ever decide to do some motion graphics with transparency.
Either a pair of video files or a pair of bitmaps for each frame would
work.

yes their is a way. if i understand the question.

but, do you know alot  about photoshop or did you just start using it?
this is important to know to help you faster.

this is the simple answer to the problem.

open the file, save the color image as a BMP and ajust the settings, 
go to image>ajustments>threshold and set it to 255, go to
layers>flatten image, (i think that black is transperant, if its not
then skip invert) go to image>ajustments>invert, save as BMP and
select you options.

hope this work if it dosen't work then try saving it as other formats.