Thanks for the reply.
The functions I wanted to assign are not available in game.
I had to go the tortuous route of intercepting the keyboard keys from the specific device, converting them to MIDI events which act upon a virtual MIDI device, then using Spad.Next to recieve those MIDI events and activate the particular functions in MSFS.
Massive PITA to set up initially, mainly conceptually as its hard to understand why it has to be so convoluted, but it works and its easy enough to add extra functions now. The other benefit is I still get full keyboard usage on the other keyboard device.
For anyone interested I used this tutorial: Tutorial: How to use any secondary PC keyboard to control MSFS events and variables via SPAD.neXt and MIDI commands
I also found this video to be helpful as a quick guide to doing what I wanted in Spad.Next: https://www.youtube-nocookie.com/embed/JC6qfUrr1tE