I've been looking at those overboots that are for sale, typically some neopren thingy (similar stuff is used by bikers in cold weather). If my climate had been colder I would have bought something like this.

What I'm looking for is an overboot that a) is large enough to accommodate my big leather boot (a leather boot made for super telemark 75mm binding is BIG), b) also works when you take off your ski and start messing around in the snow. Those neopren thingys will act as upside-down funnels and squeeze a lot of snow in between the overshoe and the boots. Not optimal.

A neopren overshoe with some rigid straps under the sole could work... Or one overshoe in combination with some "slippers" that you put on when you take off your skis.

My biggest problem with skiing in the cold (not talking about -40C here, it doesn't get that cold where I live) is that everyone's feet will get cold during lunch, or while we play in the snow, or while we dig that cozy snow cave. Keeping your feet warm while on the move is much less of a problem.

My cold weather skiing has been in plastic shell double boots designed for mountaineering and compatible with ski bindings. For really cold condition, I was carrying full nylon and neoprene over boots that would accept crampons but not ski bindings. On this trip (Denali), we spent a lot of time at a camp at about 14,000 feet. We wore the classic "mickey mouse" over boots which were unfailingly warm and comfortable, down to -80F windchill, the coldest I have ever been. Those, unfortunately, will not play well with either crampons or ski bindings.