Yeah I've been thinking about doing something along those lines to determine the FOV of my finder. It'll certainly help with star hopping.
As for the idea of getting a straight thru finder; I've toyed with it, but the fact is, I have to wear glasses to see anything (astigmatism) through the scope. I'm also short sighted so I can't use a straight thru finder with both eyes open to do 'reflex' sighting, because I can't position my head effectively so that I can see thru a straight thru finder and the sky at the same time with my glasses on. My neck doesn't bend that much!