I need to add a Mojave partition to my native Catalina iMac 2020, 27", T2.
To date all attempts to boot from a Mojave external install drive have failed. "Secure Boot" is set to "No Security" and "External Boot" is set to "Allowed". Catalina just shows a "No Entry" sign after selecting the Mojave Installer drive. I know this machine will support Mojave. I am able to boot exclusively from a Catalina external install drive. If I wipe the computers internal hard drive from a Catalina external install drive will I be able to access the machine and install Mojave? If so how do I proceed? Any other possible scenarios. What about target disk mode?
Your iMac shipped with Catalina. Generally, Apple does not support using an OS older than what the device originally shipped with. Yes, there are the occasional exceptions. Unfortunately, your model is not one of them.
If you really need to run Mojave on your system, then you should probably consider running it within a virtual machine.