I have made a first attempt at a skeleton plugin for reading XSPF files (playlist file format), that I make available to anyone who wishes to develop a plugin. A sample category tree, as expected by the plugin, is as follows:
<catgories><br />
<category title="entry 1" type="video"><br />
<url>http://localhost/misc/test.xspf</url><br />
</category><br />
<category title="entry 2" type="PlaylistGroup"><br />
<catgories><br />
<category title="entry A" type="video"><br />
<url>http://localhost/misc/test.xspf</url><br />
</category><br />
<category title="entry B" type="video"><br />
<url>http://localhost/misc/test.xspf</url><br />
</category><br />
<category title="entry C" type="video"><br />
<url>http://localhost/misc/test.xspf</url><br />
</category><br />
</catgories><br />
</category><br />
</catgories>
Feel free to improve this as you feel necessary.
Edit: Reuploaded the file, noticed a few bugs