There are hobby wireless projects that provide the radio on both ends (instead of using BT MIDI) that’s one way to allow simultanious wireless and DIN MIDI connections.
Hairless MIDI is an app that runs on various platforms, including 32 bit Linux x86 or x64, they reference a terminal counterpart ttymidi
GPIO Pins are a limited resource, improvising another serial connection seems challenging, apparently these bridges work with USB as well. Just using a little board with built in USB-MIDI seems like the simplest means to make a wireless link without added support code, that doesn’t have the overhead of BT MIDI.
(The ttymidi page references QjackCtl a GUI JACK patch manager that I imagine would foul up the Zynthian routing, he has several other Linux music projects)