Actually, while you can't short SF medallions, I do know that can short a company that primarily finances NYC medallions.

Look at the symbol TAXI.

http://www.google.com/finance?q=taxi

From their profile:

Medallion Financial Corp., through its subsidiaries, operates as a specialty finance company in the United States. The company is engaged in originating, acquiring, and servicing loans that finance taxicab medallions and various types of commercial businesses.

So while it was probably hard for you to short subprime mortgages directly in 2008/9, this is the same type of thesis. Short the company that has made loans on an asset that is quite likely going to depreciate in value over time. Just like you could short the banks that made the bad home loans. Given a long enough time horizon, it seems that you'll do well. But check with your own financial advisor.