That's about as easy as it gets I'm afraid. The functionality you want would be external to Blitz so you can't really avoid having to either write a "bridge" dll (for CallDll) or knuckle down and define a .decls file (for more fully integrated variable passing).
Incidentally, if you ever see an example of the Modplug playback library being used give us a heads up - nobody seems to know how to implement it. :/
Incidentally, if you ever see an example of the Modplug playback library being used give us a heads up - nobody seems to know how to implement it. :/