I would suggest your last sentence "Would they be ok inside the house on a window sill in a room that's not used and gets lots of light." would be best. My spouse has one in an office with a big window and it does fine but not as lovely as the real thing outside. I would suggest being careful with watering as they thrive outside in places like Phoenix AZ where watering may be sporadic. Search the entire Gardenweb for more answers. There was a Canadian link to growing bougainvillea as a houseplant but I cannot get it to work. Perhaps some of the Canadian people here on Gardenweb remembers what the site reported.

Here is a link that might be useful: Link I cannot get to work. Perhaps you will have better luck

Hi, I'm from Phoenix and we have lots of Bougainvillea, frost usually won't kill them. They loose there leaves and go into shock, but don't die. I'm not familiar with a cold greenhouse but if the temperature doesn't go below 35 degrees F for too long you don't have to worry. You could always bring them in on really cold night. Here, we just put a sheet on them. If we forget, the leaves turn black, but the plant survives. Keeping them moist usually encourages plant growth, but the bracts (pink flowers) come about when they are distressed (dry). So the secret is to keep them a little on the dry side if you want the flowers. I think more Bougainvillea die from over watering so I'd be careful with that. It's easy to tell the difference the leaves will start wilting but recover quickly when watered. They really need full sun (5-6 hours or more) to flower well.
Nancy