I wouldn't say that-- the Wii had excellent homebrew/softmods. If all the same stuff can be done on the Wii U, even in just Wii Mode, that'd be pretty cool. Sure, we wouldn't be able to do HD graphics or the Game-pad, or any kind of Wii U related stuff, but Wii homebrew is a saving grace to the Wii for some people. Maybe people could get more out of the Wii U too, even if it is just the same emulators, free homebrew games, or even backup loaders (thinking in the grand scheme of things there)
It certainly is worth investigating at this point, even if you will never get any Wii U access in this sand-boxed Wii-Mode. What I do know is we don't know enough about the Wii U's Wii-mode to make any strides in editing the Wii Mode's NAND with any kind of custom code. Hell- we don't even know if Wii-Mode has any write access to the IOS at all. (My guess would be no- i assume Wii U mode would handle that side, but who could tell at this point.)
One more thing to think about. If we hackers are capable of patching Wii Games on the fly, who is to say Nintendo couldn't create a patch for brawl?