Need to manipulate the content-type of a return without access to the code

Question

An off the shelf application is setting a 302 back to the browser with an invalid MIME type. Our reverse proxy is dumping the return due to the invalid MIME type.

The backend is IIS 6.0 and I have been looking at URL rewriters But they only appear to match the incoming stream not the outgoing stream.

I note that IIS7 has the ability of setting outbound rules.
So do I have any options other than fixing the backend code or going IIS7