That's unfortunate. I thought you hadn't already started on Silver.
If I remember correctly, there were someone else who asked for something like that and it got denied, but I could be wrong.
There are still 2 more options in my mind which you could do:
Since every player is allowed to have 4 accounts until the merge comes, you could do another one, play it until you have 4 badges, trade the mount to this account and change server with it, so you can trade yourself the mount again on Silver
Give the mount to someone you trust, who wants to change from Gold to Silver and this person gives it back to you on your Silver account