Yeah, except... if .mptm is supported directly, then each separate sequence can contain 255 orders and patterns, and then you could indeed make use of 4000 patterns (how many sequences can there be?).
Then seeking different sub-tunes could be done by seeking Sequences instead of Offsets within the sequence. That would be very convenient indeed, since there would be no need to convert the .mptm back to .it.
Hmmmm. Interesting indeed.
But this would have to be implemented by the authors of BASS.DLL and FMOD.