Are the frequencies unique though? Allthough highly unlikely, what if someone had one of those and walked by? I guess it would be one in a million but still.
I've been looking for the same thing for a while.
Now with Christmas coming up I'm trying to find an exact solution so I can "suggest" it to family.
This was the most likely thing I found in my search:
(look at the bottom product on the page - " CS-8051 "Hands Free" Immobilizer System")
I called this company about 3-4 months ago asking about the product (notice it's now "coming soon" at the bottom - that wasn't there before) and they told me it was still in development. They weren't sure about an exact timeline; call back later.
Since we're both looking for the same thing I'd really appreciate it if you let me know about anything you find. I can be reached at strix (dot) varius (at) gmail (dot) com.