In Bassmod, there is no direct command available to change the tempo. If there was I would have wrapped it.
What may work is putting tempo commands on the rows of an empty track. Then when the tempo must be changed, you first back up the current position and row in variables,
then set the playback position to the appropriate track and its row, then play a single row and then change back to the song position you were in.
Alternatively, you set the tempo to immeasurably long let your game decide which position and row must be played at what time, so you can let the game decide when to play something. However, since your game will be in charge of timing, this requires your game to keep up perfectly well with the screen sync.
Wouldn't the first method cause weird bumps in playback? It might cause interruptions in the current track, and besides, I'm under the impression that it automatically sets the initial tempo defined in the IT file when you start a new sequence.
And as for the second method... Since the game I'm using it in has a strange propensity to lag, that's not very... practical to say the least.
Don't know how FMOD handles it, but since it's a very mature library I assume it should work just as well as the Bassmod library I am using. As for DLLs being liabilities... your computer has dozens of them loaded and running at any time.
FMOD doesn't seem to be able to change the position of IT files very well (changing sequence position doesn't seem to work at all), and DLLs in general are liabilities.
No, FMOD doesn't seem to be able to recognize sequences in IT files. And I meant GM DLLs, not WIndows DLLs.