Fair. I think my ultimate preference would probably be to have the string validated against the parameters. So, something like:

    (define-http-get-route ("/hello/:who" who)
      (format #f "<p>hello ~a</p> " who))

Prevents the confusion of people that remove a parameter from every path it was in, without also removing the parameter it would have bound to. Though, I suppose you would probably need to do a bit more to allow moving from query to path and such?