In order to charge the IMA battery, you would need much more than just a 12v solar panel. You would need a DC/DC converter to raise the 12v to the level of the IMA (144V for a HCH-I) and you would need to know where to connect this voltage source to. Can't just connect it to a NiMH battery without some kind of Heat/Current/Voltage protection circuitry.

It can be done but not without some re-engineering.

I believe that in the HCH-I, the IMA battery is used to start the engine by way of the IMA motor. The 12v battery/starter would very rarely be used. I've owned my for 2 years now and it is parked outside in the cold NE winter and the 12v starter has never had to kick in.