I think for Quattro I will use a dual power supply. A CR1220 for the RTC and a LiPo for the rest. I got some ultra thin 3.7V LiPo's with an integrated protection circuitry.

• 40.8 x 30.2 x 3.8mm
• Protected against overcharging > 4.25V
• Protected against discharge < 3V
• Rated voltage 3.7V
• Capacity 500mA

Should fit nicely on one side of a 50 x 50mm PCB together with the coin cell holder, the charging circuit, USB connector and boost converter: