Only the early versions of Brawl allowed this, any newer (which was not even half a year after release i think) fixed the glitch in the music file. This required us to take advantage of the ungodly easy to hack wii lol....which honestly is the sole reason i hacked mine. I wanted brawl+. i didnt even put NES/SNES emulators on the thing for a loooong time, despite being a good platform for classic gaming.
3DS might be hackable but unsure how it would affect online play. We already have a modification for capture cards that dont deny online, so never know.
WiiU is unhackable unless you want to remove it from the internet. Its option to auto update is a lie, it will update whether you want it to or not. I believe its just as easy as the wii was to hack but again its that autoupdate problem, not the hacking itself. Which is the best way to deal with hackers imo lol.
My thing is though, the Sm4sh game is fine as it is. Couple of tweaks could be needed, but theyre minor. Nothing as huge as OMGWTFTRIPPINGZOMG! crap lol. Brawl had a LOT of flaws in it, thats why modded brawl became so popular.
Someones going to try to mod sm4sh, mark my words. Question is, what do you sacrifice to do it and was it even worth it?