Currently (as you mentioned), MTS offers the fireplace as a free video on demand rental. For technical reasons, it makes more sense to offer it as a Video on Demand rental (VOD you pretty much upload the video and the information, set the removal date and price and it's live. A TV channel means you have to edit the programming guide list/data, have a server setup streaming the channel 24/7 (even if no one is watching), upgrade channel mapping/grouping so that the channel is included in something like a basic group so everyone can get it, then push an update to all the set tops to update their information -- then, because it's only for December, have someone go back in the new year and undo it all).
On MTS Ultimate TV, VOD appears in the Guide as a dedicated channel, so once you order it you could flip back and forth to it. (It automatically pauses when you change off the channel, and resumses when you channel back).
Regarding an HD fireplace, I don't suspect so this year. At this time, MTS does not offer HD VOD (the system is capable of it though), which is likely how this would be presented in the future given all the work that would need to be done to create a new channel.