Is it plausible to use a thumbpick for very fast alternate picked sections, and for sweeping, or is the angle created by the pick always being against your thumb not right for these techniques?
Look up Chet Atkins, thats about as close as you can get.(That I have heard)
Actually, I read in a Betcha Can't Play This column in Guitar World over a year ago that Chris Broderick had started using thumbpicks a lot when he played, because it allowed to do large sweep-tapped arpeggios very fluidly. So I guess if you practiced it, and were the spawn of Jesus, like Mr. Broderick is, that it would be plausible.
Hell yes. Went to see Tommy Emmanual about a month ago. He shredded up some alternate picking and sweeping to my surprise.