If you know the Tempo and it does not change; then it is simple maths to convert Beats and Bars to Frames. And someone may know a neat way to import such data into AE.
However, inaccuracies will occur because (AFAIK) KeyFrames are applied to the start of a Frame, which won't necessarily occur exactly on a Beat. And it gets very hard if there are changes in the Tempo.
You may find that only having the Beats is inadequate. Rather more fun is to have all the Notes, Channels, Velocities and durations.
Although not designed for AE, the Windows program "MIDIANAL" http://www.tima.demon.co.uk/midianal.htm may be of some little help. And an updated version might be available, on request, from the Author.
I guess I will make a wish request on Adobe.com
for a "bpm synch layer", I'm SURE a lot of people would like that.
doesn't sony vegas (which evolved from Acid, a music prog)
include such things?
(even if it does in rendering get rounded to the nearest frame,
which yes, sometimes is half-way through a beat!
but viewers are unlikely to notice.)
I guess doing everything at 60 FPS, or some other frame rate,
and then re-mapping it back to 30 (yeah 29.97 whatever)
might also help.
as you can probably guess, I'm making music videos for myself / others
(in which I often often DO have richer data sets available)
on youtube as VJFranzK
and also getting into more video remixing aka "mash ups"
(where I DONT have anything but the clips and approximate tempo.)