Compatibility does not restrict creativity, Saga. Not in any way at all. However, a module format that is not supposed to be able to use VST, should not be able to use VST. If such a thing is to be implemented, it has to be done in a new format, or there must be a header to indicate which version of the format to use. If ModPlug did this, there would not be any problems.
Oh, and Pailes, I must correct you on your VST statement. There are actually players that support VSTs in modules. I don't know how many, but PlayerPro allows for VSTs, both on MacOS Classic and X. It also happens to come with a bunch of VSTs to my happy happiness and has a bunch of standard VSTs allowed in the PlayerPro custom format (A very versatile format. Too bad the tracker is not working too great. But the sequencer works pretty OK.).
(Just noticed Pailes replied the moment I pressed "reply".)
Hmm, OK. Yeah, this made my post pretty obsolete. :>
EDIT: Oh, and I believe Renoise, although Renoise is not a player, allows for VSTs on MacOS X.