So I created my first node! Not entirely sure if it doubles up on functionality of others (Apologies if so), but I had to get the ball rolling somehow.
It’s under package “Atmos” and the first node is called “XY Frame Extents” - I’ve done testing and it appears to be robust.
You simply feed a Select Model Elements node into it and it will give you the frame extents of your selection as a rectangle, the corner points and mid points.
Hopefully this is useful to people - and any improvement suggestions would be more than welcome.