Alright, nobody been down this road recently? I'll share what I've found as I go then. Maybe somebody else can get something valuable out of this.
I found an online paint source that still carried the mythical late '80s GM Dark Blue Metallic paint and sold it in touchup quantities for only about $25 shipped to my door.
Not wanting to commit it to the bike just yet, I shook the bottle thoroughly, applied a wet coat to a piece of scotch tape and let it dry overnight. I cut the tape with scissors so I had a representative patch of the paint right on the edge and then applied it to the bike. This is the result:

Maybe that's close enough for some people. Maybe it was mixed wrong. Maybe I picked the wrong color. I'm going to say that in really small dots (stone chips) this might not be noticeable but, anything larger and it's going to be obvious.
I'm calling it a bust and moving on to either buying a pint of properly mixed PPG paint or finding a place that can match the paint on the bike.