This RT extension (designed for RT 3.4.5 and newer) implements a per-ticket "Wiki" whiteboard field.

To install it, type:

    perl Makefile.PL
    make install

The extension looks for a custom "wikitext" field called "Whiteboard" which applies to a 
ticket being displayed. This way you can easily have different whiteboard fields with
 different ACLs on different queues.