An update on this issue…
This issue is definitely related to the similar issue affecting CS4 installs created with the CS4 Enterprise Deployment Toolkit.
If you mount the disk image in kernel and notremovable:
sudo hdiutil attach diskimage.dmg -notremovable -kernel
You can now successfully install from the disk image. Of course, you can’t detach the image and must reboot to eject it, so this is a poor workaround. But it indicates the bug is similar to the one for CS4 installs.
For the CS4 installs, I was eventually able to find another workaround; so far similar ideas for the CS5 disk image have failed.