location of RSS XML files

There is no rss xml file that is stored on the server for the wiki. The wiki generates the rss feed when you ask for it. The way to get an rss feed for any page listing is to add .rss to the page name requested.
So, if you want an rss feed of a particular search:
/groups/mygroupname/search/index.rss?q=search_term&field=title

So, you have your web based rss reader do a GET on that url to get the rss data. You can do the same thing with atom by exchanging .rss for .atom

I would be grateful if you expounded on that "generates" statement. I realize there are XSL transforms going on, and one can override these by duplicating them from /usr/share/wikid/apple_templates/ into /Library/.../WikiServer/Themes/. But what is being transformed?

Best I can tell, Apple's customized python code is building XML on the fly and handing it off the the XSL files. I looked through the /usr/share/wikid/lib/python directory, and there is so much going on there: zanshin, twisted, xmlrpc, sqlite3. Hard to reconstruct how all this works.

After another day of looking & experimenting, it is apparent the XML is generated by the classes in apple_wlt (subdirectory of aforementioned path). This doesn't mean I understand it enough to customize it, however.

The rss and atom responses don't go through the xslt. In generating the rss and atom, there is no intermediate xml format. It gets rendered straight to the format from memory.
Rather than try and answer this question more, I'd rather ask: What is it you are really trying to do? Asking the right question will yield much better answers.

I'm in a similar situation. In the user blog pages, I want to pull additional information about the user and display it on the page along with the blog posts. I'd like to make a query against some other MySQL tables I've created and add the results to the generated XML. Can you tell me where I should be looking?