How do I subscribe to RSS feeds in Safari on a page showing the Reader button?

Safari (the desktop variant) shows an RSS button in the address bar for any page with an associated RSS (or Atom) feed. Clicking that button opens the RSS feed in your chosen feed reader application (Safari by default).

However, on most pages the RSS button only appears briefly as the page loads, or not at all. When the page is fully loaded it's replaced by the Reader button. There doesn't seem to be any way to disable the Reader button or otherwise get to the RSS button on such pages - so how do I subscribe to the RSS feed for the page (other than hunting around for an RSS link in the page content or worse, the page's source code)?

Click and hold the Reader button, and a menu of available feeds appears.

(Discovered after much Googling at Mac OS X Hints.)