And that is indeed the N/A issue.
RSS can't access the authenticated directory. And there doesn't appear to be a way to put a relational URL (the menu requires a 'valid url' - 'http://something.ext'). It's an external feed and insists on being external.
I tried creating an RSS user and passed the username/password to the site via the URL in the RSS url , which sorta worked, at least got me past the folder encryption, and that changed the 'Site': -> N/A to 'Site': -> Tiki Error: You don't have access to these pages, (something like that).
That error message also shows on the Wiki page.
Adding the RSS 'user' to the Forum as a registered user got me nowhere.
(I suspect the RSS URL would have to have the viewers Tiki username/password or I have to allow anonymous viewing - and anonymous folder access - if it's to work at all. The user/pw URL thing may actually be fighting with security protocols and this idea is a dead-end no matter what I attempt.)
So, the one hanging question (academic now, perhaps) before I attempt to follow Luci's advice is whether the RSS code is theoretically proper in the Wiki page directly, or whether the RSS code should actually be in a module and the module in the wiki page.
If nothing else comes from this it's been a fun (?) learning experience.