You just need to take a screenshot, transfer it from 3DS sd to Wii U sd, and use WiiU-Shell. I posted a pic on that method. The best we have until Pretendo support.

not done yet :,(

they are working in acnl and ssbros online so wait until they support it

Last I checked, Rosalina menu does come equiped with a screenshot function, it's the very first on the last

*list