Been their, done that...
It's not as bad as it sounds, you do however need a warm change tent/shack/bus. Take along dry clothing that you can get into in a hurry such as sweat pants and shirt.
The hardest part of diving wet in winter, is getting out of the water. Getting in, and staying in is fine. The water temp will be the same as it was below the thermal in the summer, so a 7mm is more than suitable. Just make sure you plan it well, and have that warm shelter to get out of your wet suit as close to the hole in the ice as possible. If you have a large prospectors tent, and can have it set up over the entry hole and heat it, that would be even better.
Have fun, and let us know how it went.