I've just found a webpage which may have some interest for those seeking to deal with the condensation vs heat loss trade-off: http://letok.narod.ru/zima.htm
Google does a reasonable job of translation, but if there any Russian speakers out there, there are a few words which could benefit from being tidied up !
Basically, what the guy has done now for 2 winters, is to completely seal the top of the hive using a layer of medium-thick polyethylene sheeting, with a thin layer (3mm) of polyethylene foam over. This is kept in place by means of an oversized eke, which holds the poly and foam in place in the same way as a drumhead is held down onto a drum by a rim. This combination allows the poly sheet to remain cold enough for water to condense on it's under side.
Then - and this is the simple but ingenious bit - he simply places a length of 2x2 under one side of the hive to allow the condensate to trickle down one of the walls.
I'm fast coming to the conclusion that there will always be a trade-off between moisture removal, and the amount of heat retention preserved - that is, unless/until some genius comes up with a really clever ground-breaking idea.
I've just realised that there are 2 sheets of polyethylene involved (dodgy translation ...), and this is how I visualise this system operating:
Black lines being the polythene, and the red line being the foam. Moist air escapes through the gap in the lower barrier, only to condense on the upper barrier. Condensate then runs down along the polythene sheet, down the inside front wall of the hive, and exits via the entrance slot.