Thanks Jeff. It sounds like a selection set is what I'm after, but incidentally when I try and do it in this case I get this:

Any idea why error occurs?
I would clean up my project file and share it but I'm slammed at the moment (can happily do this later).
To answer your question- the reason I would like to do it this way is to be able to develop a procedural workflow (to sketching) as I iterate my designs. I come from a CG background so I'm used to building things procedurally and in groups, so that I can easily go back to a source group (or math formula) and translate/scale/etc.
For instance:
One thing I've not figured out how to do is create a formula within a sketch, so that I can dimension each line according to a formula. I've found that Fusion will do this in a basic sense (simple addition, multiplication, etc), but as soon I try and get more complication Fusion breaks down. I.e. I want the length of an edge to be a percentage of the length of another edge, while that edge is calculated by subtracting the length of one edge from another, then divided by two... etc. The goal is that my 'source edge' can be changed and everything else will be resized formulaically. But, I've discovered that as soon as I get more than one-relationship deep in Fusion I start to get errors. (And forget about using projected geometry in a formula, I'm still wrapping my head around why that is so temperamental.)
Anyway, I'm just trying to finding the boundaries of what Fusion can do, and what workarounds there are doing procedural type things that make life easier when you want to make changes later.
Cheers.