Been a good while since this project had an update. I was waiting around for my paycheck before I ordered a few of these prototype boards. They're finally here, and they look great! Had to ditch the castellated holes on this run of prototypes since it hiked the price a good deal. 

(soldering this together was a larger task than originally expected)

These boards seem to function pretty well under all the tests they've been run though. Currently they produce a clean 5.3V on the output whenever the battery voltage is between 3V and 4.2V. The external circuit supplied by the boost converter has its ground disconnected in the event that the voltage falls outside of that range. On all major points these boards function properly, however there was one minor issue with this board. Mistakenly the MCP73811T was used in this project which does not have programmable current, driving the PROG pin to USB-VIN sets the charge current to 450mA. A simple fix: 

1. Connect PROG to USB-VIN to set charge current to 450mA
2. remove R4. 

This is just a bodge, really this entire issue could be fixed by simply switching out the MCP73811T with the MCP738112T which has the programmable current or fixing the schematic to reflect this change. 

I'm also not entirely sure about the polyfuse in this circuit either. It does prevent any serious over-current damage to the rest of the circuit, but there's a lot of room for improvement. If I go ahead with a redesign I'll definitely be looking for something to replace it. 

That's where the project stands at the moment, still have some work to do with it. Luckily the first prototype works fairly well.