There are many different types of solder with different melting points. Typical 60/40 solder has a MP around 370 °F or 188 °C. That sounds hot... but I guess that would melt in a typical cookstove.
Wikipedia has a list of typical solder alloys & MP.
http://en.wikipedia.org/wiki/SolderI assume electrical conductivity on the backing plate is a bit of a problem, but is there a way to bond the cells to an insulator, then to an aluminum plate? Adding fins would be a bit excessive, but perhaps you could increase the surface area by adding a second aluminum backing plate with half punched up fins...
Or...
how many panels are you repairing? Could you thermally bond scrap automobile radiators to the back?
My Siemens Panels have splurged some goop out from around the contacts... something that I'm assuming that decreased their efficiency. I don't know exactly what caused the problem, if it was heat, electricity, or both. I could try to upload some photos of the cells later, but some (older) cells do not do well with too much heat.