Well, that happened quickly. Since the last log, I have added a #SOICbite Programming/Debug Connector Footprint and a couple of mounting holes, but other than that, this is basically the same circuit as in the First Stab log: 

A few advantages I hope will work out - the programming connector's VCC is protected from feeding the supercapacitors by the D2 Schottky. Also, if the MPPT thing doesn't work out, I can always just leave the solar panel connected to the supercapacitors via the ATtiny (I plan to use solar cells which won't exceed the ATtiny's max voltage or current through a pin). 

I need to be careful that the mismatch between the voltage at VCC (dropped by the schottky Vf) and PA2 (no drop) doesn't cause a catastrophic backfeed (15mA is the max rated in the datasheet) by accidentally turning PA2 to OUTPUT when the ATtiny is running from the supercapacitors only.