The old way to do this was to put an alias to the locked iso into the OS9 Startup Items folder (in the System Folder). Make sure the iso is associated with Toast (type and creator), so that if you double click the iso it opens Toast, can't remember if it mounts it for you.
This used to work with aliases to .img's.

Option 2 is to try a little Applescript and put the script in the Startup Items. Not sure how scriptable Toast is.

Anyone else ?

Thats what I was thinking writing an applescript then put that in the Startup Items......I'll have to mess with that when I get a chance....
Actually I just tried applescript and does not do so good with toast...However I just tried the alias of a toast img in the startup folder....Once the system starts you will see toast pop up and then just hit mount....Its less steps and its a reminder to mount the img..

Don't use Toast. Use the "Virtual DVD-ROM/CD Utility" from the MG to do the mounting. Test that it will mount your .iso (it will mount most files that are .iso image compatible, not only .Toast, .cdr etc).
Using ResEdit or "Creator Changer" also here at MG, change your .iso's Creator type to VCDM and its File Type to GImg (case sensitive). Save and then test by double clicking the .iso - it will auto-launch the Virtual CD utility, mount the .iso and then also auto-quit the utility.

If the above works OK for you, then place an alias to the .iso into the Startup Items folder inside the System folder. It should mount at each boot-up until you remove the alias from then onwards. This is what works for me anyway.

[BTW] if using the Virtual DVD-ROM/CD Utility to mount the .iso, do not lock the image as the utility mounts .iso's as "read only". So if you lock the image the Finder will only nag you about the image being locked when launching it, giving you an unnecessary button to click off.

@MTT +1