A problem I've had for a while too. My batts are in a cabinet outside and when it's below freezing I run into the over-voltage problem, too.
One solution available is to adjust the C40's bulk and float voltages lower, to say 14v/13v for the rest of the winter. On really cold days set it lower. You then need to be sure you equalize every month. When you do equalize you need to disconnect the inverter.
The other thing to do is to disconnect the temp sensor altogether. They you're stuck turning the little knobs on a very regular basis!
One thing that should reduce your worry - cold batteries do not decay and sulfate as quickly as warm ones, so you can live with partial recharges for longer without permanent damage. But the equalization become more necessary to make up for it, or else what you think is an 80% charge will only return 60% of the Amp-hours that you expect, and it will get worse the longer you let it.
Last week was really cold here, and very windy. I ended up with my C40 turned down to 27v/26v so that the temp-compensated voltages wouldn't go over 28.2v. That's the upper limit of my sealed AGM batteries. I can't equalize them and they can't be allowed to vent so I'm stuck with even lower voltages than you have.
