Ok, I think I might live to regret this, but I'm now leaning toward maintaining two separate sets. I just don't see a decent scale factor that will work with what we've got. We've got snap values to think about, text sizes, ... I dunno.
I think it's going to be two separate sets, drawn on the appropriate snap, saved in read-only directories. If someone wants to modify or add a block, they'll have to come to me to do it, so I can do the same thing in each set. I know it sounds like a lot of work, but I think the result in each drawing will be much better.
