Turns out you have to have an NFS share for your SGE_ROOT directory. So I set up NFS.
I followed the GUI instructions from this one. In short, you add an /exports entry to NetInfo with settings for the directory you want to export. I couldn’t get the exports to show up right for a long time, but restarting the machine fixed that problem.
To set up an NFS automount on the PowerBook, there’s some more setup to be done, described here. The gist is to set up a NetInfo entry in /mounts for the server.
End result: my /usr/local/gridengine on darwin.local maps to the same directory on astor.local.