How about running a separate web server on the same machine, just to the kill requests? You could set up this web server to only accept requests for the specific script or directory for the kill script. As long as both servers run as the same user, I don't think there should be any problem for the second server to kill processes of the first server.

I don't know how you have the kill script set up currently; maybe it just kills jobs running under the same server. With the proposed second server, the kill script could present a list of all the jobs running on the first server. You'd pick the jobs you want killed, submit the form back to the script, and the script would just kill those jobs.