Thanks for the numerous answers. I've just spent about a day and a half experimenting around, and here's the good news.
Brasero is now a full-fledged replacement for K3B and integrates nicely in XFCE!
This required a lot (!) of work, though.
First thing, install the whole lot of multimedia libs and backends: lame, a52dec, xvidcore, ..., ffmpeg, ..., transcode, etc. If you're curious about the details, take a peek in my SVN:
$ svn co svn://svn.tuxfamily.org/svnroot/microlinux/slackware
All the details are in the ChangeLog.txt file.
Once this is OK, install all the gst-* packages, as previously suggested by kingbeowulf.
The SVN repo also contains a rewritten SlackBuild for a more recent version of Brasero.
Last thing: if you install totem-pl-parser and add the --enable-playlists option to the build, Brasero can even handle playlists from XMMS.
I've tested it for the best part of this afternoon, and it works perfectly, plus it's a lot less resource-hungry than K3B.
Here's what it looks like on my build box:
Cheers from the sunny South of France!