Developing \$C_{out}\$ spec for DC Bench Power Supply
My question is: Is Cout part of the plant or part of the compensator?


I'm favouring C\$_{OUT}\$ is part of the plant but it's of no consequence to the analysis. I say it's part of the plant because it isn't actually needed for stability given that you are using a 2N3055 in an emitter follower configuration. It could be argued that it is part of the modulator of course.