I just don't see why Nintendo would. There's simply no gain for them. It wouldn't make SMG2 sell significantly more, maybe 500k, maybe, M&S combos are getting old hat. Why bother with the legalities or revenue sharing?
The only way I could see this is if in exchange for Sonic in SSBB, they licensed Mario for M&SatOG and now in exchange for Mario in M&SatWOG Nintendo has the right to use Sonic again but nothing better to use him in. I don't think we'd want Sonic in Zelda or Metroid. Kirby.... maybe.... Really, Mario is worth 10x's what Sonic is and this really only helps Sega any way you slice it.