Tied legs or arms would be a nice quirk but being tied to something would be too risky for me especially with large breeds when some minor body adjustments seems inevitable to avoid any potential major fractures, ruptures, scratches or injuries in general. When you get literally a huge bone with even greater bulge inside, a tiny change of angle may move it slightly inside which can change awful pain into awesome pleasureI would not be keen on being strapped down.. I like to be able to move with the dog and to get the most out of his excitement.
I feel that being strapped down it will lose a lot of the wonderful interaction you have with a dog when he mates with you.