I think I've got it.
The Finder info of an old disk image that auto-opens its root directory:
00000000 00000000 00000002 00000000 00000000 00000000 5A156DD7 DA8B129A
The Finder info of a newly created read-write disk image:
00000000 00000000 00000000 00000000 00000000 00000000 28F57114 97D99395
In both cases, the frOpenChain field (the 4 bytes starting at offset 20) is zero, so that doesn't seem to be what's used for this purpose. One clear difference between the two is the frLocation.v field, the 2 bytes at offset 10. That appears to be the iNode number of the folder you want to open as a Finder window, represented as a bigendian 16-bit number. Setting that does seem to do what I want!
Topic:
Developer Tools & Services
SubTopic:
General