I have managed to get one mode on the Mayflash NS2 to cross from Wii U, to vWii (only in USB Loader GX at the moment), to Nintendont.
It only works on the Mayflash NS2 Red LED "Switch" mode. It may only work with the NS2 unless I get more info on the NS and S adapters to see if they have a matching mode with matching HID data. It won't work with the adapter in Purple LED "Switch Pro" mode, or any other mode.
It took a custom ini for HIDtoVPAD (alpha version for Aroma) and custom ini for Nintendont, and a custom compile of Fakemote installed to a cIOS in slot 247 (in my case) along with a custom USB Loader GX forwarder channel that uses the Fakemote IOS to launch the USB Loader GX homebrew app.
Obviously you can't turn the console on from anything USB, so you still need the gamepad or or Wiimote to turn it on.
In Wii U mode, no Mayflash attached controller can trigger the home button in HIDtoVPAD. There can also be some contention between the USB input and the Wiimote/Wii U Pro controller. It appears HIDtoVPAD isn't informing the OS to move them to player 2. Maybe
@Maschell will pick up development again.
In vWii mode, I have not applied the fakemote patch to the menu IOS 80, and all the d2x 249-251 IOS installs, so right now it only works in USB Loader GX using a forwarder. The WiiU USB Loader GX forwarder also works, but still requires the modded wii forwarder. From there it works in Wii games, and Nintendon't. A side note: There is a bug in priiloader right now that doesn't let you set a custom IOS for the home menu.
No mode can tell the Mayflash attached controller to change LEDs, rumble, or use an accelerometer..
The patched version of Fakemote is really, really ugly. I took the PS4 controller code, stripped it down, and modified it to work with the switch mode, I MIGHT be able to get it to cooexist with the other modes it supports (PS3/PS4/Dragonrise), but I really have no idea how much memory is free for such stuff on the vWii arm cpu where the USB ports connect. Maybe
@xerpi is still around and could answer that.