Huh - learned something here. Our service people have always said "don't do it" when we asked about this years ago. I just took their advice at face value, and didn't investigate further. I guess they worry that it wouldn't be done correctly, so "no" is their default answer.
Still not going to do ours, because our gas bill (heating, hot water, and fireplace) is only $60Can a month. But good to know if we ever move, and things change.