You could add an item to the F8 menu, but that is really not publicly documented. Possible and not hard, just tedious and no public docs on it.
Here is something you could do easily.
Deploy a Secure PXE Representative. This uses SYSLINUX and now you can modify the syslinux.cfg\default config file to provide an option.
Now you can go to http://syslinux.zytor.com/wiki/index.php/The_Syslinux_Project and find all the information you need to provide an option to your users after they select WinPE.
Just edit the syslinux.cfg\default to provide both options and you are good to go. You can remove the password if you want, you can do almost anything you want.
They can use LANDesk's WinPE or your company's LiteTouchPE.
Just take the time to read the documentation.
If i had my way, LANDesk would only have one option in the F8 menu, and all other options would be handled by SYSLINUX.
that is one thread we had a discussion on modifying pxe menu to boot your own syslinux items. syslinux is awesome and can boot many things including .img, and .iso
take a look at that discussion and may help you on the right path of booting your .iso if not and you have any questions just ask, lots of smart minds in this forum!