Ya know, something like that would be a simple thing for a welding shop to make up. Threaded rod welded to a disc. Krylon rattle can paint to match the wall. Easy peasy.
Don't forget that there are double-female threaded rod couplers. You could cut the "stem" of our mushroom back inside the wall, screw on a coupler with perhaps a jam nut then use a regular bolt on the TV mount side. But if looks on that side aren't important just hang the mount on the threaded rod protruding from the wall, sock it up with a nut and washer then cut the excess length.
-Hal
