I'm coming to like Brasero more than I did before (hardly used it in the past to be honest), and now I'm wondering, are the plugins that come with the default install the only available plugins? I don't remember seeing any in apt, so I guess they would have to be plugins you download, compile and install (fun fun fun :P!).
Well pending any answers to the contrary, I'm going to say it: No There aren't any out-of-tree plugins available for Brasero at the moment.
I've just had a 10 minute hunt around lead me to the solid conclusion: Brasero needs better documentation!
Its gnome site is rotting (says the latest version is 2.30) and doesn't contain any useful development guide for making a plugin, let alone where to find others.
The latest help page is for V2.2.
Gnome-Files doesn't have anything for it except the core project.
The mailing list appears to be dying. It's down to ~1 thread per month. You might do well to ask on there as you might catch the eye of a core developer.
If you want newer plugins (I don't think any of them are new, they're just newer versions), grab the latest tar from their FTP and compile it up. I can't guarantee any form of stability. I'd say if it ain't broke, don't try to fix it.