Ste,
So what you are talking about is the folder tree view to the left - and not the table to the right where you only see one level at the time? Because in the one to the right one I think you *can* add IBA's as described above. We have not done it on folders but on documents.
If so it looks like something that is hard to do. Of course you can always enable the JCA debugging and see if you can somehow implement your own custom version of the OOTB classes if they are . E.g. If you append &jcaDebug=true to your folder infoPage e.g. http://yourwindchillhost/Windchill/app/#ptc1/tcomp/infoPage?oid=OR%3Awt.folder.SubFolder%3A44456920&u8=1&ContainerOid=OR:wt.inf.library.WTLibrary:44456024&jcaDebug=true you can see the class names PTC use.