Hi Dave-
Thank you for your considered response. No, your watermark solution is not quite what I had in mind! 😉 -- But it's a whole lot like how I'd have approached it back in the drafting table days: lay out grids/guides for the plan & 1 or 2 elevations, slap a piece of trace over the top and start drawing! Except if I understand correctly there's no using the lines in the watermark for inferences, right? So not exactly a step forward...
From your 4th paragraph I'm thinking maybe I need to learn about layers -- which I know something about from 2D CAD and Photoshop but am barely aware of in SketchUp. Might I create a lattice of guide lines (x, y, z grids) or other geometry in a separate layer?
I'm in the weird place where I don't use the program enough to get fully on top of how to use it, and the more I use it the less I know...
Anyway, thank you for re-addressing my question. I think I'll look into layers and see where it gets me!