To mount Yellow Toe Nail on Fleabite Worm, I drilled two holes into the wyvern's shoulders, to insert two 3mm magnets.
Placing two magnets on top, I bent paper clips into shape to mount them on these second magnets.
After securing the paper clips in place with super glue and miliput, I bent them into shape and cut them to length to mount the orc, into which I also had drilled two holes.
After gluing the orc in place, I added more miliput for stability, before removing him.
Not looking nice so far, but working. I can now use Yellow Toe Nail on Fleabite Worm, remove Yellow Toe Nail and use the wyvern on its own, if the rider gets killed, and have a Yellow Toe Nail on foot in case the mount gets killed during a game.
The magnets in the shoulders I hid under a layer of miliput / grees stuff, which I textures as the area around it.
The connection of the rider got smoothed out with more green stuff / miliput.
Next job is to add nice optics, to hide the functional mount. I think of sculpting a saddle under the orc, then hide the rest with draping down pieces of cloth.