See these:
http://www.ppbb.com/scgi-bin/boards/986/museum.pl?noframes;read=207508
http://www.brackenrig.net/OBC_Hack.htm
http://www.whiteson.org/boxster/mods/obc/
It's a pretty easy DIY Hack (took me under 1 hr.), but you have to get the dealer to turn the OBC on beforehand using the PST2 or PIWIS diagnostic tools. There have been other reports of dealers being reluctant to downright stating it's not possible - untrue. Keep persisting, find another dealer or independent with the diagnostic tools listed above. Good Luck!