I know there are thumbscrews available for expansion slots, case cover and hdd, BUT are there thumbscrews available for attaching motherboard to case standoffs?
Yes. Take a standoff with you to your local hardware store. You can get threaded machine screws for a few pennies each. I get mine at Ace Hardware. They have bins full of assorted screws. The selection is much better than at the big hardware stores.
EDIT - They won't be actual thumbscrews like the ones used to secure side panels to a pc case. You won't be able to screw them in by hand. I don't think I've ever actually seen thumbscrews used to secure a motherboard.
^Yeah, it's hard enough to get a screwdriver in around some places the screws go already, I cannot imagine trying to tighten them, or loosen them with my hands!
lol exactly what I was thinking ^.^ If you need to take the mobo out so frequently, got one of those cases that has a mobo tray, or just use your computer on a test bench.