I chose to use an Arduino compatible MCU that I had in the lab an 8bit Atmel megaAVR. I had thought of a breadboard PCB for prototyping cutted ad hoc with an ATMega328P (total cost: 2 Euros)
Everything looked beautiful, but in the end it was too thick to enter with all the others components in the ball. In most I was forced to add an external voltage regulator. At the end I folded on the excellent Arduino Pro Mini 3.3V 8MHz 328 which uses the same processor in SMD version and has a regulator up to 150mAh
A negative factor could be the minimum voltage requirement of Arduino Mini set at 3.5V against the LiPo minimum supply voltage of 3.0V. The external regulator handles this condition and is able to provide 3.3V both in StepDown (4.2-3.3V) and StepUp (3.3V-3.0V). In fact after a full discharge test by monitoring the voltage of the LiPo I have verified that a voltage below 3.5V is achieved after about 95% of the discharge time. So, no problem.