The repair guy in Alamogordo fixed my AV receiver! He never heard from Onkyo about a replacement board, so he did some digging online and apparently it's a fairly common failure, common enough for their to be repair tutorials. He dug in with a soldering iron, replaced some caps, and IT LIVES!
$60, plus my original $20 deposit. Seems fine. The only glitch putting it back together was one of my speaker wires broke -- at the speaker! easy enough to fix with a pocket knife. Plus apparently the digital port is on the amplifier board, not the HDMI board, so if it happens again, I could buy a HDMI switch with an optical out and bypass the HDMI board entirely.