You can modify the Safe Margin in the Project Settings > General > Action and Title Safe Areas.
There's no plug-in type for creating custom overlays for the Source and Program Monitors. I'm not sure what your idea is, but perhaps could achieve something similar using an effect plug-in. Effect plug-ins built using the After Effects SDK can have "Custom Composition UI", so that when the effect is applied to a clip and its header is selected in the Effect Controls Panel, it can draw over the video in the Monitor. You could create an adjustment layer and apply the effect to that clip, so that you could have an overlay over the entire timeline.
Interesting... though that definitely wouldn't allow me to implement what I had in mind, at least not practically speaking. It would work great for a one-time situation but not day-to-day stuff.
As for modifying safe margins... is there any method for including custom images (png, etc) as overlays? The menu you're referring to doesn't appear to have a setting for that. I'm basically looking to block out certain "safe" sections of video with an overlay for quick checking within my workflow.
Thanks for the answer, I appreciate it!
Custom overlays for safe margins are not supported at this time...