Thank you tsante!
I first wanted to try your suggestion on my iMac, so I did this:
1. Made a copy of the .toast file and put it in another folder.
2. Changed .toast to .dmg
3. Double-clicked the .dmg file, and it opened Toast, which mounted the file!
4. Quit Toast and unmounted the disk image.
5. Did a Get Info on the .dmg file, and even tho the file said .dmg, Get Info knew that it was really a .toast file.
6. Used the Get Info panel to change the Open With... to something else. There was an app called "DiskImageMounter.app" and I selected it.
7. Closed the Get Info panel, double-clicked the .dmg file, and it mounted the image!!!
8. Did a Find File for "DiskImageMounter.app" and found that it was in CoreServices which is part of an old Mac OS that I had squirreled away. It was NOT in my current Mac OS 10.5.6, but I figured that there must be something equivalent in the current OS that does the same thing.
9. I ZIPed Toast and the DiskImageMounter.app, and trashed the originals.
10. Double-clicked the .dmg file, and "something" mounted the image. Although the Get Info was still set to Open With "DiskImageMounter.app" I don't think that was what opened it.