I tried to create something that sort of mimicked the iMUSE system in Dark Forces (and many other LucasArts games of the DOS era). Works pretty decent in testing, though it takes a bunch of work to set up per song.
Timestamps (each about 5 seconds before the transition):
First track plays for 45 seconds or so, then I re-start the vid to demonstrate.
Transition to combat music loop at 1:43
Transition to intermediate at 3:48
Transition to main loop at 4:50
It's not perfect but I think it's pretty neat, regardless.
Music is the original DF music using a Roland SC-55 soundfont.
