Wii Controller passthrough
You could connect Wii-motes directly to the Steam Deck's bluetooth using pass-thru mode from the Dolphin emulator but the Steam Deck's onboard bluetooth only seems able to connect two wii-motes reliably. If you're looking for 3 or 4 player you'll need another solution. The Mayflash Dolphinbar may be that solution.
The Dolphinbar should work out of the box, just by connecting via USB to the Steam Deck, via a USB adapter or dock. You want the Dolphinbar in mode 4.
The problem will be is that SteamOS tries to take control of the Wii-motes leaving them unavailable for Dolphin. So you need to blacklist them from SteamOS.
You'll need to confirm the hardware IDs of your Wii-motes. Plug the Dolphinbar into a Windows PC and head to Control Panel > devices and printers > MayFlash Wiimote PC Adapter properties > hardware > HID controller properties (4 options, check any of them) > Hardware IDs. Its likely to be a VID/PID of 57e/0306 but its worth checking.
Head to Desktop mode on your Steam Deck and, if not done already, disable SteamOS read only using the following command into Konsole...
sudo steamos-readonly disableAfter that head to /home/deck/.local/share/Steam/config/ and open config.vdf in KWrite.
Scroll to the bottom and look for...
}
"SteamDeckRegisteredSerialNumber" "xxxxxxxxxxxx"
"CSettingsPaneGameController.Timeout" "#SettingsController_Shutdown_15"
"ControllerGyroCal"
} You'll want to replace this with...
}
"SteamDeckRegisteredSerialNumber" "xxxxxxxxxxxx"
"controller_blacklist" "57e/0306,57e/0306,57e/0306,57e/0306"
"CSettingsPaneGameController.Timeout" "#SettingsController_Shutdown_15"
"ControllerGyroCal"
} Then reset your Steam Deck.
From Dolphin check you've "Emulate the Wii's Bluetooth adapter" checked, all Wii remotes are set to "Real Wii Remote" and "Continuous Scanning" is ticked.
And you should be set to go!
Known limitations: The Wii-motes connected to the Dolphinbar will no longer drive the SteamOS, so you'll need another controller to find your way around; The speaker on the Wii-motes doesn't seem to work.