In particular, Slashcode runs http://www.slashdot.com (a high-traffic, high-visibility site), and is highly reconfigurable: enough that you wouldn't recognize the externals.
This is exactly the argument that I made with a colleague for an Intranet/Message Board/Knowledgebase. We are adapting Slash to this and it is working out quite nicely. In fact, we are eagerly anticipating the new release which will include an automated graphic upload so that our tutorials have a less kludged process for uploading screenshots. THAT and the new plugin architecture is rife with possibilities.... something not in Slash? No need to throw the baby out with the bathwater, just code a plugin to add the functionality and still keep all that is right with it. Great software package and development architecture.
As far as making the pitch to management. I wrote a proposal and did a walkthrough of some popular Slash sites (Slashdot.org, of course, as well as Plastic.com). The selling point had to be the University of Utah's Engineering labs computer site (http://www.cade.utah.edu/). They use Slash for very similar purposes to ours and have modified the heck out of it.
Update (4/19/2002 2:45pm EST): Added link to University of Utah's College of Engineering Computing Facility