Not all org files are destined for export. I slightly feel users should be allowed to put citations in places where it might not make sense in export, and that they are responsible for knowing what they are doing.
I am sympathetic to the reality that the second half of that statement is a big ask, and that it can lead to confusion. Nothing stops anyone from manually typing or pasting a citation into those places though. I would be inclined to use the activate function to highlight those that are in places that won't export, rather than limit where people can put them using the insert mechanism.
John
-----------------------------------
Professor John Kitchin (he/him/his)
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803