I've been getting more and more unhappy with the long list of stuff I
don't want to see in the system category pane of RB.

http://de-1.tric.nl/~cg/mc/Tric-RefractoringBrowser-CdG.6.mcz (or any
later version) is a quick solution to this. It is a subclass of
RefactoringBrowser that operates on a filtered view of categories, and
has menu entries in the system category pane that allow you to add and
remove stuff from the view. Also, if you navigate to classes by e.g.
'find', their categories are added as well.

You can start a single one with 'RefractoringBrowser open', if you
like it make it the default (I didn't know this, but the browsers'
window menus have an entry for this).

Hth,

Cees