Here's what I got to work with DropDMG, with the help of Michael Tsai from DropDMG:
Start with (or convert to) a read/write .img file on Mac OS 9 (Disk Copy on OS 9 can convert a regular .img file to a read/write .img file.
Then, on Mac OS 9, mount the writable .img file using Disk Copy and put all the icons where you want 'em.
Then, mount the same writable .img file on Mac OS X using Disk Copy, and again, arrange the icons the way you like them.
You can go back and forth as often as you want using OS 9 and OS X, arranging the icons (if you have nothing better to do, haha). Changes you make on each platform don't affect the other one (disk icon positions are stored in a completely different way).
When you're all set with the image file on both platforms, convert it to NDIF compressed on Mac OS X. I used DropDMG's "convert" option but I guess you can do the same thing with hdiutil -- I think that's what DropDMG does anyway.
I was able to end up with a self-mounting disk image that will self-mount on either platform and the icons are arranged nicely on either platform (not necessarily exactly the same, but nicely).