While we have the Movie > Show Input Display and Movie > TAS Input options, neither of these are particularly good as an actual input display. I'd like to have one that looks similar to the one on
http://gamepadviewer.com/ . The Input Display option isn't really intuitive for watching, and the TAS Input while better and has a stickbox as opposed to the Input Display's purely showing the stick values, an actual input display would be really nice since it seems from any of my googling nobody has been able to hook into the Wii U adapter passthrough like dolphin does.
What are you needing it for? Is GPV not doing what you need?
GPV doesn't read the controllers in passthrough mode (using the wii u adapter option in the controls menu), and while it might register in directinput mode using the standard controller option, and the PC mode on the mayflash adapter (what I'm using), the mayflash adapter's PC mode seems to have weird issues in dolphin, with being unable to calibrate the sticks correctly, at least for me.
What I'd like it for is just an input viewer whenever I happen to be streaming or recording something in dolphin, specifically recording as for the types of videos I record, it'd be helpful and clearer what I'm doing if an input viewer were up.
GPV hasn't received an update since 2017 that's why it still uses the old school ways (still deserves an applause for making that tool). Someone in the OBS forum shared another open source
gamepad viewer. It uses updated gamepad API for connection and works great for all 3 of my controllers. But, I don't have a Wii U controller so I am not sure if this would work with that, still I think it is worth a try. hope that helps.