Ficus hates to be moved! They typically drop their leaves in response to any change in conditions. I think you have to find some compromise spot where it can just stay until it is time to bring it outdoors in the spring. How cold is the sunroom at night? If it stays above 60 deg F, the ficus can just stay there. Otherwise you may have to sacrifice the best lighting for somewhere that it is warm enough for it to stay. If you need to put it in a warm, but not well lit room, you could provide supplemental lighting for your bonsai.
Supplemental lighting is fluorescent lights hung just a few inches above your tree, on 16 hrs a day.
For one tree, this can be done with desk lamps, as long as the whole tree is lit from only a few inches away.