I'm wondering if this could be accomplished in conjunction with an Endpoint Security agent. So my GUI would just run normally as the logged in user, but would be "protected" by an Endpoint Security agent that would trap "kill -9" calls. Could that work?

OK. And what triggers that situation? Could be a number of different events. Could be an unlock event (com.apple.screenIsUnlocked). For context, I usually see folks implement this sort of thing in terms of screen unlock, and you can do that using an authorisation plug-in. However, that’s not something you can reliably trigger on demand. We are specifically not interested in using an Auth Plugin because we want more flexibility than is offered by the Auth Plugin API (e.g. no biometrics, very prescribed UI). So the first nut we are trying to crack is this issue of "unkillability" by the logged in user. I'm hoping to get an idea if this is even possible. If macOS 15.* and onward simply do not allow a GUI to be launched by anyone other than the logged in user then I am probably out of luck. Thanks for your help!

Thanks for the response. The word I was trying to write was a-s-s-a-s-s-i-n. I.e. a process whose job is to kill another process. I'm not interested in running a GUI as root per se. What I want to do is have a GUI that takes over the screen (or screens) and prevents access to the user's desktop unless they perform certain authentication steps. This doesn't need to be a root process, it simply needs to be unkillable by the logged in user. If there were some way to run setuid(X) where X is not the logged in user, that could work.

The above should read "via an assassin shell script" - not sure why Apple replaced it with "*".