Q: Is it possible to compose a good sounding song completely in locrian?
A: regardless of good or bad, it's almost impossible to make a song that actually sounds like it's in locrian. because if you confine the entire song to that one set of notes (G locrian, for example) and stay in it for long enough, it's going to start sounding like the corresponding more normal mode (Ab ionian, Eb mixolydian, etc)