Just found out about bless.
It is a command that allows you to modify aspects of the filesystem. Like flag a volume as bootable. One nifty option is --openfolder.
From the manpage:
--openfolder directory Specify a folder to be opened in the Finder when the volume is mounted by the system.
So I first tried changing the current setting to another existing folder
sudo bless --folder "/Volumes/Douglas" --openfolder "/Volumes/Douglas/var"
After unmounting and remounting the volume, the Finder automatically displayed the var folder this time. You can’t specify a nonexistent folder, so I went ahead and created dummy inside of tmp on the Install DVD partition, and then issued this command:
sudo bless --folder "/Volumes/Douglas" --openfolder "/Volumes/Douglas/tmp/dummy"