The Pyblosxom plugin I created works quite well. If there is a file called doap.rdf in a category the plugin will create replacement variables so that my flavour templates can insert a <link> element in the header and show a button in the sidebar. These links point to the file, so of course at some point a client will try to load it. However Pyblosxom treats this as a request for the article with the ID "doap" using the xml flavour. Thankfully plugins can catch requests for files and respond to them there and then, so the plugin can handle these requests manually.

End result: the Sound Juicer blog category has a DOAP file, so the category has a <link> element and a link to the DOAP in the sidebar. This is more than enough for the DOAP Viewer to find the DOAP, as can be seen here. At some point this week I'll clean the source up and submit it to the Pyblosxom plugin registry, but if anyone wants it now just mail me.