Redirecting in Mongrel

I'm setting up a website with Mongrel. One of the requirements is for
URLs to be redirected to different files than they usually would,
e.g.:

http://example.com/users/show/123-king-kong

Would actually look for the cached file:

`public/users/show/123.html`

And not:

`public/users/show/123-king-kong.html`

as it normally would.

How would I set up such redirection with Mongrel?

Thanks in advance,
Maurice B. Gladwell