I’d like to be able to draw dashed borders around some components to denote high-voltage or sensitive circuits. Can we add something like this under the Miscellaneous category?
Generally you can draw a polygon on the “Guide” layer to draw a box around a circuit. It’s just not possible to use a dashed line (see Support dashed/dotted polygon outlines · Issue #812 · LibrePCB/LibrePCB · GitHub).
Or if I misunderstood your question, please clarify 
Ah, I didn’t see that earlier request.
This was in reference to schematics only. I don’t think it would translate well for Gerber output, I’d leave that out.