> Are there any stations that are going all-Christmas on the
> main channel, and putting their normal format on an HD
> subchannel?
Not that I have heard of, but I would expect that at least a few stations that go all-Christmas may indeed put their "regular" formats on an HD subchannel.
Conversely, there may be a handful of stations that don't go all-Christmas on their main channel that will mount an all-Christmas format on an HD subchannel.