A lot of my info about the FMV stuff comes from experience fooling around with utilities released by NEC in their PC-FXGA package.
30FPS is definitely supported, don't get me wrong. It's just that it was virtually never used, and the quality takes a hit when you're at 30 FPS with a full screen image and max audio quality (31khz stereo if I remember right). However, I did actually experiment with encoding a 30FPS TV show, scaled to the proper resolution, and the quality didn't suffer too terribly.
Bear in mind that the system is updating the TV screen every 1/60th of a second without fail, and it either repeats a frame or draws a new one. Thus, the framerate has to be evenly divisible by 60: 30, 20, 15, 12, 10, 6, 5, 4, 3, 2, and 1. Something like 25 FPS would actually be fluctuating between 30 FPS and 20 FPS very rapidly. This is like what the Tengai Makyo game does, fluctuating between 12 and 20 FPS IIRC.
The intro to Battle Heat uses a constant 30 FPS only during the moments that you see rocks and water. The rest of the time, it's fluctuating like Tengai Makyo. It's very windowed, though.
Hardware gurus: if the PC-FX FMV was 256x240 12FPS, what were the specs for HuVIDEO?
Bonknuts might have to weigh in....
You could actually check this yourself with Mednafen pretty easily. Open up Gulliver Boy or whatever and take a screenshot. With that, you can measure the resolution. Then turn on frame-advance mode and see how many frames it takes for the picture to change. I bet it would take 5 or 6 frames, which would mean 12 or 10 FPS, respectively.
EDIT: Just checked. It's actually 10 FPS, at 192x112 resolution.
12 FPS is sort of the standard with cheaply produced animation, whether we're talking Dragonball or The Simpsons. It's a nice number because it can be doubled to fit the 24FPS film standard, or quintupled to fit the 60Hz NTSC standard. That probably has as much to do with why so much PC-FX stuff is 12 FPS as anything.