A bit over a week later, I called for a status update, and eventually got through to the repair person (who, it seems, is usually out on service calls) last Thursday morning, and found out that he was as stumped as I was. I figured that I should take the new motherboard back to the store I bought it from, and have them check it to make sure it was ok, since the 30-day warranty was about to run out. This led to a small circus involving me trying to get to the store at the same time someone was there to let me in, and confusion over which motherboard was which, and eventually yesterday after lunch I was able to meet him in person and pick up the right board.
So, I drove over the bay to Oakland (20 miles), and brought it to the store. "We'd be glad to test it out for you. But, er, we don't have any of the right sort of memory, and it would take us a couple of days to order it. Is there any chance we could borrow yours?"
Er, right. So, back across the bay (20 miles, $2 bridge toll), and found that the technician had just left on another service call. Luckily the owner was there, and she let me go back to his workbench and pick up the memory.
Back across the bay again (20 miles, rush hour traffic), and I handed the motherboard and memory to the store owner, and walked with him back to the workbench.
His technician hooked up the board (sit motherboard on a small box; hook up a two of the three power leads and a power switch; plop a heat sink loosely on the processor), and just as expected, it didn't boot.
So, he poked around at it for a bit, and amongst the poking, he pulled the CMOS battery out of its socket. I was rather dubious -- the manual described a proceedure for clearing the CMOS with a jumper, and I'd done that without it fixing anything -- but I was quietly dubious, and said nothing.
And, lo and behold, it booted.
Admittedly, it crashed a half-minute later, but that problem was quickly fixed by picking up the heat sink, putting a bit of heat-transfer compound on the CPU, and putting the heat sink back ("splut"). And they poked a bit more, and confirmed that it was running correctly and recognizing all the memory, and handed it back to me, with the explanation that sometimes the CMOS gets mixed up when one changes the CPU or memory, and that it needs the battery removed to clear it and let things boot again.
So, back over the bay yet again (20 miles, heavy rush hour traffic, and $2 bridge toll), and I got to the repair shop 10 minutes after they'd closed. Not that I was expecting to be earlier, really, so I'd already arranged to bring the board by this morning -- which I did, but again the technician wasn't there, so I just dropped it off and then emailed him with the details of what had happened.
Grand total: $90 for the motherboard, two weeks of waiting and playing message-tag, 80 miles, two bridge tolls, and miscellaneous unaccounted effort. All for one CMOS battery removal and replacement.