So here's where things stand at the moment.

The flex PCB works and I can communicate to the SI1133 from the Sensorwatch. UV light does make it through the LCD and plastic to the insides of the watch. I have a working 'movement' watchface to show a raw ADC count of UV light

But there are a few things that don't work or that I don't understand.

 1. Interrupts don't work. I suspect this might be a problem with how the sensorwatch runs when it's plugged in via USB (faster clock, no sleeping), although I haven't tried since I switched from a 'raw' app to a movement face

 2. The code doesn't handle when the sensor overflows/saturates

 3. The SI1133 has a few different filters/mode, and I don't really understand the difference between UV and UV-Deep (or the IR / Visible modes but I care less about that). Posted on the SI forums for an answer

 4. There's still no code for calculating UV index because I don't yet have a method for correlating raw ADC counts (at whatever gain settings!) with actual UV hitting the watch (in Lux?) or with the actual UV Index

 5. I don't have a good feedback loop, it's a pain to open the watch up every time I flash new code. I think I want to solder some wires to my spare SensorWatch and hook it up to a programmer while it's still inside the watch body