I'd like to use a "bare" Li-ion battery pack like this:
It seems like it would be relatively easy to solder this inline (somewhere) but I don't have the abilities to know where and what the gotcha's are. It would also be nice if I could still charge this battery pack using microusb (built-in or otherwise).
Is this a simple thing?
The gotchas are in the charging. Lipos are charged at constant current for phase 1 of the charge until they reach ~4.15 volts/cell (usually at 1C or less - so a 4000 mAh pack would be 4A or less - although there are now batteries that can be charged faster, they are heavier) and then constant voltage (~4.1-4.2 v/cell) with decaying current until current is about C/100. You will need to incorporate some electronics to achieve this - or simply detach the pack and use a dedicated lipo charger and learn how to care for them.
The other gotcha is in the discharging. If you discharge right down to 3.7 V/cell (resting voltage) or below, you will shorten the life of your batteries. So you'll need some circuitry for that too (a warning buzzer or a cutoff).
Ignoring these can be dangerous as these cells will go bang if abused. Mobile phones etc. all have this stuff built in, and even then, occasionally you hear about one going pop in someone's pocket. I don't know where to get the circuitry from, as I have lipo chargers and stuff already from my electric RC planes. But I'm sure it is not hard to make or obtain.