Thanks all - I do have a stencil, but the 2D array of pads makes it much tougher than the edge-contact ones I've tried before (can't visually inspect, and can't fix with a soldering iron), so a lot of my issue is "how do I know if I've got it right".
Gordon's method ended up being the most successful - the squeezed-out solder balls inspired a lot of terror ("if solder is coming out the side, might it not be squishing across pads internally?"), but everything seems to be working. To mitigate my own terror, in my next version of the PCB I've also omitted the pads I'm not using, which is almost certainly a mistake for a variety of reasons, but its made layout much easier and I can only learn by doing :)
Attached is a shot of the latest version, with Micro-USB charging. I swapped some pins and setWatch isn't working on the ones I swapped it to (11, 28, 30) - might be a soldering issue again, but just in case the next version goes back to 0,2,3,4,5,29