Dumb is actually a worse choice than Modplug, as Modplug is mostly intended to be a player library, but also is used for a tracker; Dumb is just a player, so it would likely require extensive modifications beyond the magnitude of those initially needed to fit Modplug into the program.
We've made a large number of changes to Modplug's engine with the purpose of improving IT compatibility, and Schism plays IT files significantly better than Modplug or Mikmod. Also, a while back some work was put into a replacement engine written specifically for Schism Tracker:
http://schismtracker.cvs.sourceforge.net/schismtracker/pm/
The main goals for pm are to be bug-compatible with IT playback, efficient, and written cleanly (i.e. no hacks). In theory when pm is working right, it should be possible to drop the whole Modplug directory from the source, cut out all Modplug references, and replace them with code in pm.
Unfortunately it's been sitting there for a long time and hasn't had much attention since it was first committed to cvs, so I don't think it's altogether likely that it's going to be replacing Modplug any time soon :(
