In a sense, I think that all creative pursuits are served well by the type of "brute force" you're talking about. You write/play/draw something, decide if you like it, and then work from there. However, narrowing down the search space, as you described, can be easier if you leave it to the unconscious. If you've been listening to the style of music you're composing in, or reading the style you're writing in, etc., it should have at least a few heuristics already loaded up. In the case of your melodies, what happens if you just "hum something?"

I generated a segment by pianistic experimentation (rather than from my aural imagination)

This is how I write most of my music. Every once in a while, a melody or chord progression will just show up fully formed, usually on the subway or in the shower, but it's rare. Most of the other songwriters/composers I know work similarly. You say you don't hear new music in your head; a lot of the time, I don't either. I hear it as it comes out of my mouth or speakers in real-time.