Are you wanting to leave the split ridgeline attached to the tarp?
If so you then your idea is great and you could use a tumble hitch http://www.animatedknots.com/tumble/...matedknots.com
It's easy to release and you could put a toggle in the bight to keep it secure.
Or just splice or tie on a dutch hook on the end of each of your ridge lines which will hook onto the line wrapped around the tree and you just tighten up with your prusiks.
A soft shackle would also work but I tend to stay away from them because when its cold or wet out they don't tend to be very cooperative with my digits.