After a whole day of scratching my head looking for the perfect charging circuit + step up + battery monitor online (while working 9 to 6), I decided to brew my own.
The circuits that I liked the most are:
Sparkfun LiPo charger basic : https://www.sparkfun.com/products/10401
Sparkfun Fuel Gauge : https://www.sparkfun.com/products/10617
Pololu Step-Up 5 V voltage regulator : http://www.pololu.com/product/2115
Adding these boards together should work pretty well for my needs (which are steady 5 V @ I >=300 mA, fast charge rate @ 500 mA, low battery sensing), but them together with the battery cost more than half of what I've already spent of hardware for this project, take so much room, and I really don't want another USB port on my device just for charging it... anyway space is what concerns me most right now; luckily, these circuits are all so small and only need a few passives to work, that putting them together on a PCB will be a piece of cake. And hey, the first two have free schematics online, thanks Sparkfun!
So, as soon as I have further time to work on the project, I will la down a PCB for this module while the other components get delivered.
As soon as I have the components, I will also evaluate the option of a main PCB that may actually host the battery management circuit, something like an "undershield" for the Pro Trinket; this may also help with keeping together all the small modules that are not provided with mounting holes.
See you next time!