Usually the RSS feed is on the same domain, so you could simply whitelist automatically. If a user adds example.com/rss you'd add example.com to the whitelist.

Usually the RSS feed is on the same domain, so you could simply whitelist automatically. If a user adds example.com/rss you'd add example.com to the whitelist. If you expect any kind of traffic you'd probably be refreshing the RSS feeds server-side on a schedule. You could then change all the URL's to signed URL's to use with your proxy (e.g. nginx securelink, ngx_http_hmac_secure_link_module and multiple other modules for nginx).