I would think that the PageBuilder plugin should be able to do the job.

Suppose you want to have an .opml file for every category. You can just define a new 'category type' page, and set

'build directory' => whatever you like
'build filename' => <%CategoryID%>.opml

This will build files 1.opml, 2.opml, etc. in the category you specified. Alternatively, you could also have your .opml files built in the actual category directory. In that case you would need to set

'build directory' => <%Full_Name%>
'build filename' => myopmlpage.opml

BTW: With the pagebuilder plugin you can build any kind of file on your webserver --- at least as long as the file contains ascii text. So you can build xml files, opml, txt, php, css and so on.

Ivan
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Iyengar Yoga Resources / GT Plugins