Having some way to limit access would be good.
For example, by having an access URL like:
http://123.34.23.244/rss.php?u=username
the php script would validate that the request come from an IP allowed for the account 'username' and print the rss xml if it's ok, and nothing if not.
I don't know if io httpd & php extension has a variable for the request IP..? I think that would be the only blocker to such a script.