Consider that the official emulator for DS games on the Wii U Virtual Console didn't have any networking support at all (and that the Wii - and by extension the Wii U - is perfectly capable of locally networking with the DS) it may well be a tall order to implement LAN crossplay between hardware and emulator.
Remember that Echoes of Time, a game that supported Wii/DS crossplay, only allowed a Wii to join a DS online, where the full Wi-Fi standard protocol was in practice. In LAN mode, the DS has to join the Wii.