You guys bring up a good point, here's how I think of approaching a solo over a tri tone prog;
Think of the root chord and the tri-tone chord as being the same chord. For example, say where in C.

C (ceg) tri-tone F# (f#a#c#)

combine the two chords together and what do you have?
A C7b9#11 chord
Both locrian & lydian will work, but they both leave out the major 3rd, my scale of choice would be a dominant diminished scale; C Db Eb E F# G A A#(Bb) C. I think this scale captures the essence of the chord progression which sounds diminished in nature, not diatonic.