This article is from the Mac Programming FAQ, by Jon Watte h+@austin.metrowerks.com with numerous contributions by others.
You stuff the dirID you want to show into the lo-mem global
CurDirStore, and the NEGATIVE of the vRefNum you want into the lo-mem
global SFSaveDisk.
If you are using CustomGetFile and return sfSelectionChanged from an
"init" message handler, you must remember to clear the script code, else
the selection will not change.
 
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