Cool idea, I mess around with things like this too.

For the most part, Jazz uses extended tertian chords and 7th's are considered the "default" sonority. Metal (depending on the specific style) tends to stick strictly with power chords or basic voicings, but is usually a little bit more rhythmically complex.

I believe you could do a few different things to combine these genres, yet still make it effective (there are more things than I will list though):

1. Use the swing feel of Jazz and apply it rhythmically to a metal piece.

2. Use the extended tertian chords and other jazz-like sonorities, but play them in a tad bit more aggressive manor accustomed to metal (you may have to turn the distortion/gain down).

3. Juxtapose or Stratify different (or the same) thematic material played in each style respectively.

Hope this helps.