To add to gnarly's comment and to be more specific, which version of MM to you have, and please explain a little more about what you mean by "powerchords, which aren't very good for playing melody".  Power chords is what metal is all about.  If you have power chords backing your metal song why would you not be able to play a melody or lead line behind them?  If you think about for example Iron Maiden.  Often 2 guitars play power chords while one plays melody some times 1 guitar plays power chords while the other two play different melody lines.  So we have the guitar parts in metal music taken care of.  Now you bring up metal notes for midi.  What are you really asking?  Your midi notes are going to sound either like classical, blues, jazz, etc. based on how you shape and eq the notes.  A note is a note is a note.  The music style that it sounds like depends on how you shape, mix, eq, and master the sound on a keyboard instrument.  Likewise how a guitar actually sounds depends on the fx you apply to it.  There is no such thing as a metal note, only a metal sound.  The metal sound is derived by what notes are played in what seqence, and what types of effects, eq, etc are applied to them.  That is it!!

Pleas help me understand better what you want to do.

George