I do not believe most dogs in general would enjoy this practice. While it seems yiffy, the primary concern when having sex with an animal should always be whether the animal likes it or not.
From what I have seen docking is usually done while the dog is on his back which is a vulnerable position for dogs especially while having one of their most sensitive organs exposed.
Another thing is that the sheath is not designed to be penetrated. It is something the dog would not perceive as a sexual behavior. In fact it is completely unnatural for a dog to have anything inserted into the sheath.
There are some cases when sticking a finger in the sheath is necessary, I have never seen a dog enjoy that.
Your best option is to buy a bad dragon silicone penetrable toy to simulate the feeling for yourself.