RetroArch on Wii U has supported slang shaders for a long time, but it's really hard to find them online. I found a pack of them, though, and decided to rehost it here. Hopefully, more people will get to enjoy them now.
EDIT (11/1/2019): To use these, copy them to your SD card and then go to settings > directory in RetroArch and set your 'video shaders' directory to that location.
Unfortunately, the GPU on the Wii U isn't particularly powerful, so relatively few run full speed, and there seem to be some weird scaling issues going on, leading to slightly uneven scanlines at times and broken xBR/Hqx effects. LUTs are also wonky, apparently, as many of the LUT-based mask effects are just scaled up to fit the screen instead of tiled across it.
EDIT: I thought the scaling issues might have been caused by it rendering at 720p and then upscaling to 1080p, so I changed the Wii U's output res to 720p but it still has a bunch of ugly issues. Mask effects are completely terrible. Currently, I'm just using misc/interlacing at a 2x scale to get even scanlines.