One step forward, 2 steps back
So I started populating the board, first doing the power-areas, and the 3.3v isolated area worked well, but then 2 problems became apparent…
Problem #1: I noticed that the scheme I'd put together for isolating power at bring-up didn't take into account the fact that the board was already partly populated by the assembly. I need to make the 3.3v power go directly to the monitoring circuit, not just to the general circuit net. There's no way as it stands to check the 1v (which monitors the 3.3v supply and only switches on once the 3.3v rail is alive) without bringing the 3.3v rail generally up.
So having seen that 3.3v worked well, I chose to just install everything, then I'd make the 3.3v solder-bridge connect, and then I'd be able to check the 1.1v rail.
Problem #2: As soon as I made the solder-bridge to allow the 3.3v to break out onto the rest of the board, the 3.3v supply dropped to about 0.4v, and the LED dimmed dramatically. Apparently, I had a short. I spent some time looking for one - the parts I soldered myself are all pretty large, so I was surprised there was a short at all, and I couldn't find one anywhere.
At some point, I decided to check the chips again, and the board looks like:
Notice the SDRAM chip above the SD-card. That dot in the upper left corner ought to be in the lower-right. Seeed had put the chip on the wrong way around (on all the boards, to boot). So, after turning the air a virulent shade of blue, I took the chip off using a hot-air tool.
Unfortunately, while removing the chip, I managed to remove some of the pads as well. Blue air intensified... I think I can patch it up (although it'll be tricky) because the pads connect to other visible points on the board, but I'm not best pleased right now.
Once I had the SDRAM chip off the board, I checked the 3.3v and 1v rails again, and they worked fine, so I'm pretty sure that's the problem. Hopefully, if I can patch the five broken pads, I'll be good to go to test further tomorrow.