@jay_s_uk Ahhh That's the sort of thing I was afraid of. Thanks, I can't justify the expense atm but you've saved me wasting money - I guess I'll save up and do it right 
You've gotta love these patents/licences that restrict open standards and in the stop people using their own products - genius.
I'm looking at switching to the stealth-burner tool head for my Voron and it lends itself to using CAN bus but I'd like to keep using my Duet 2 Wifi + DueX5 expansion.
A previous discussion explains why the duet 2 can't do CAN-FD natively but I've found the Seeed studio makes a CAN-FD module which uses the GD32E103 processor. It uses AT commands over a serial link to send CAN messages.
It would connect to a Mellow Fly-SB2040 V2 on the tool head.
It appears to also have SPI connections, its not clear if that can be used for CAN messages or if only the serial interface can be used.
I'm happy to make changes to the firmware to support this, I assume theres a lot of work to create a new type of "tool" but I'm not sure if its feasible to connect it.