I have thought about something like that, not anything public, just making the connection feel even deeper, and somewhat supported by something more than only emotions. I even got a set of rings, one for me, one for my boy, or rather for his collar. However I decided not to do anything like that, because as much as I loved him and as much as I am sure he loved me, it's just not possible for dog to understand this human concept we have made up, and he cannot say a clear and thought-out yes. He can easily show a consent to do anything else in relationship, he liked to be with me as much as I did, and he was most of the time the one suggesting to get intimate. But this is not something he can decide for himself and I believe it would be wrong for me to decide it for him.
TLDR: Haven't done that despite thinking about it, dog cannot give consent for marriage.