i m no specialist, so i may be completely wrong, but why not add a resistor (not too big) near the battery to force current into the mcu
this would also augment voltage since the power consumption(mostly due to battery recharge) will be lower (the overall solar pannel load)

btw i'm not sure of the serial capacitor usefulness
for me a capacitor has alwas been a insulator in term of current
- SOLAR PANEL |
| |
| |
+->---||----->-+
| |
|->|--(-BATT+)-+
| |
| |
GND Vcc

this could be a little better (you have a basic low pass filter with the capacitor)