I believe underneath the bullshit, your game purchases are still linked to your NNID (AND the system they're bound to). As long as that NNID association exists, I can't see why they couldn't create a mechanism to modify which system they are locked to via your account. I never expect it will be possible to have games in a playable state on more than one Wii U at a time, and there may well be some arbitrary limit like you can only change the associated system once per month, but doing so is plausible (and likely, IMO).