Interesting idea for a secondary board, perhaps linked into the USB. I have heard it done before via the JTAG, but I don’t believe this is available for Mayfly.
Possibly the biggest question on issues like this is power budget. If there was a reliable power budget for a linux SBC (beaglebone.org) it would be relatively easy.
One area to track this new feature request could be
Just a thought, another potential would be to look at the value of a next generation board, with that capability built into the chip. Since that ticket @220, where I’ve identified a method on issues/220 on how it could then be upgraded, I’ve been keeping a lookout for methods. I personally like the SAMD51 of which there is a lot of software support, and has the port capability for managing halfduplex comms which is the core of SDI-12.