Coming from a Kotaku interview with Two Tribes' Martijn Reuvers...

"Our dream was basically to let users create levels on whatever platform they'd prefer and that they would be able to share it through Steam Workshop. So we came up with a proposal that would allow Wii U owners of Toki Tori 2+ to login to Steam, so that they could make use of that functionality. Kinda what Valve did with Portal on the PS3.

Nintendo and Valve reacted very positively and we had some calls and e-mails with both parties about this proposal. After doing a thorough technical check, we decided that in order to create a near seamless integration, we needed more functionality that was out of our control. Unfortunately things like this take time and cannot be implemented overnight. So for Toki Tori 2+, it was too late at that point. However we still are in contact with them, so should things change, we'll definitely revisit it again."

Looks like the two companies are indeed open to working with one another, but this implementation would have been too far above and beyond the original plan. At least now the door is open!