...since you'd need a wired connection (or a magical second wifi interface in the WiiU) in order to tunnel the connection...
It would be entirely possible for it to be done on the same interface, but still, nintendo doesn't give things away for free.
Probably not, local communication most likely doesn't use 802.11b/g/n (the wii calls it "Nitro" mode) and the network driver (which handles both wired and wireless connections) can only use one mode at a time.
Are you sure about that? The wii u hasn't even been released yet, so how can anyone truly say for sure. Yes, I understand with normal circumstances, this probably would not happen, but if N engineered a driver specific enough for this concurrent multi functionality then I see it as being a possibility. I know in linux I can send out a packet of complete garbage data that does not have a header or even any typical 802.11/802.3 structure to it, all while being connected to an AP. Layer 2/3 will just reject any data that does not follow the standards, thus all that rejected data could easily be data that a 3DS could be set to accept.
Although I've never bothered to do a wireshark analysis or used airodump while transferring some shit from the DS to the Wii, i'm sure that it probably does use standard 802.11 protocols. It's not like you can't make a wireless interface associate and authenticate with multiple AP's at the same time and sending and receiving data. The only problem with doing that is that it splits up the possible bandwidth rather badly and you get a decent chance for more collisions to happen and dropped packets and such, but thats what acknowledgements are for.
Would N ever waste their time, definitely not. I honestly don't even know why I'm partaking in this absolutely pointless conversation anyways.
EDIT: After rereading what you wrote after finally getting some sleep, I think I now understand what you were saying. But as we know, the wii only has bluetooth and 802.11g, and the DS and 3DS only have 802.11b/g. There is no unknown or undocumented hardware on any of those devices that could explain what your are talking about...at least in all that I've read in the past.
EDIT2: After doing a little reading, the only thing that I can find referencing this "nitro" mode was one thing on wiibrew. Other than that I couldn't find anything within the first 3 pages of all my searches.