I've never heard of one, but a dog will cock its leg at anything upright. What I do is watch my dog as soon as I let him in the garden, with a bucket of water at the ready, pour this straight over where he/she has been and you wont have a problem.

How sad to lose a magnolia - you must be gutted. How about wrapping something round the trunk at widdle height? Perhaps something like a brushwood collar with a space between it and the tree?