Is this an evergreen rhododendron? As a landscape tree, these things can be chopped severely and will backbud like crazy if you do it in mid spring and they are healthy. I think there is a risk of significant dieback pruning away all the foliage in mid winter, though maybe not as big a deal as with conifers. I guess we'll find out. Good luck,
In a real cold climate there could be a lot of dieback, but here in Seattle area (and probably BC) not too much. I cut one to the ground two years ago in winter and it's now 4' tall, but that one's in the ground. In a pot it would be more vulnerable. Spring is best though.