YES!!!! Remote focus possible with 40D Live View!!!
Thanks all for the comments. Just discovered an interesting point - remote focusing via Live View (posted this at DPR/FM too):
Just tried it on my 100-400 IS, remote focus is possible in Live View if the camera is tethered to a computer.
1. The bare 100-400 IS can be focused remotely, no problem.
2. 100-400 + any wired TC - remote focusing is disabled, as expected.
HOWEVER, with the 100-400 + dumb Tamron 2x TC, remote focus works at f/11....Wowowowww!!!!
I suspect a taped Sigma/Canon 1.4x or 2x would also work, will try this later. It doesn't matter if AF with the untethered camera + TC hunts. As long as the camera at least attempts to AF (even at f/11), remote focus works via Live View!
Hehe.... that means moon craters jumping all over the LCD during physical MF is now a thing of the past....