I think there are quite a few companies marketing in-wall subs now. Of course - given the built-in nature of them - it would be just as good to DIY.
Absolutely agree that it will be difficult if not impossible to optimize a built-in sub, but you might be able to mitigate the room node problem my installing multiple subs in multiple locations.
Depends on how much flexibility you have in final placement. Most subs go next to a wall so I don't quite see how that is much different than an in wall at the same location.
