Hmm, turns out SEQ files use VAB files as the samples, not VAG. I'm guessing it's still the same thing with a different header. (VAB=mutiple samples, VAG = 1 sample?)
One day I'll have to take a peek at an seq file. I know my way around MIDI events, and I'll be able to reconize any if I see them. I'm guessing the VAG is appended or prepended to the file.
Then again, another thing I could do is a memory dump of a PSX, find the seq loaded into memory, and mess with the data directly.