So my first improvement was to purchase an HDMI-DVI cable to have Xbox (HDMI) -> projector (DVI). 720p worked fine (note, if you pick the "optimal resolution" option from the Xbox display menu, it calculates that only once... found that out after having "optimally" being at 1080p at tekman's and my projector then screaming at me), Rock Band had to be put in overscan mode, oh, and I had to manually switch out the DVI cables to go from computer to Xbox or back... which threw off projection angle and then I'd have to fiddle with it all again.
So this week I acquired a manual HDMI switch. Why manual? Cheaper, and less likely to have anything fancy in it that adds video propagation delay. Also, a 3 foot HDMI-DVI cable and a 10 foot HDMI cable (meant 15, but 10 luckily was barely enough after being shifted toward the console a foot by the switch).
So now I have
HTPC: DVI -> HDMI - Switch input A \ Switch output (HDMI) -> DVI - projector digital in / Xbox: HDMI - Switch input B
Two things of note happened with this new configuration:
1) Rock Band 2 no longer needed video overscan. It was saved as enabled on my console, but it seemed like things were cut off. Sure enough, turning it off made it happier.
2) The video propagation delay changed by less than 20ms. When we were first playing Rock Band 2 I had some mutterings about the video calibration which were eventually echoed by tekman and angel_grrl, and calibration came out to 20ms... since that's still the setting and I didn't feel it was off, presumably whatever the change was ended up within my tolerance. (Take it as a given I didn't become more tolerant. No tolerance for stupidity, and since I can see myself saying "stupid video propagation delay" it qualifies as stupid :P)
So it seems to work fairly well. Push a push-button and change the input mode (the projector can't auto-detect Video GBR vs Computer (need a newer one for that)) and change the audio input mode (it occurs to me that people might find the concept of needing to press so many buttons appalling, but remember, I like buttons and gauges. I wish my car had more. Honestly. Like a commercial airline cockpit... except none for the right-hand-seat. ALL YOUR BUTTONS ARE BELONG TO ME!).
Still confused where the overscan went, but hopefully projector native mode with no overscan on will mean no Rock Band vocal line video scan errors (it's so wiggy when the tambourine circle splits momentarily).