reverse engineer the CD32 FMV card.... ?

[TribbleSmasher]

The FMV decodes the video directly to the monitor, there is no transfer to memory involved (not exposed to the user, afaik), you are right about that. The only thing to add is that the native Amiga screen gets genlocked onto the output of the FMV.

With a Vampire card you do not need any decoder hw anymore, it can decode MPEG1 streams in software, see Riva player.

[TribbleSmasher]

Bandwidth is the key, if you keep it below 1500kbits, you are in safe area. The Vampire demos are all around 640x360x1500kbits.
And they play fine, there was even a x16 core for the V4 tested. If you want to decode the stream, you most certainly don't want to do this yourself, but instead use a better gfx card with FPGA/ARM, like the ZZ9000, and let it do the work for you.