It's not as daft as it sounds the Toyota Prius and the new Nissan electric car do something like this with regenerative braking and also speed controlling going downhill on cruise control. But it only recovers about a 1/3 of the energy generated in the battery. I would suspect had the C10 been produced it would have done that too, but the C5 is too small to be any use at it.
To use solar panels, well you could make up a bracket and fit 2 off 100w panels with a nice big 30 amp diode, but that would cost more than the C5, but it could be done, Sinclair gave one to the Sci-fi writer Arthur C Clarke with them fitted. If you fit your 12 V Socket you could get a solar charging station of about 60-80 Watts that would charge you up quite nicely after short runs of a couple of miles if it's parked up for a few hours. The little 2 or 4 Watt ones would take for ever to do that.