(Sorry if its already been suggested i didnt really find it in forum though)
I know might be probably a bit specific/only few might use it idk. but its something that i never knew i wanted til i had it:
i’ve also been using a programm called Umake which is more like a CAD type program for doing buildings and such. Anyway, tied to their camera-views they have a Markup (the included art-editing tool that comes with the apple devices) overlay you could view or hide within the camera-snaps. you could edit the work with with the markup visible in front or just have the realtime change visible when you worked on the model from a different angle (which hides the markup) then snapped it back to the saved cameraview. This is a lot different to just using a reference in the back or taking and editing a screenshot maybe even nice to take mental notes. no idea how much bandwidth that would eat or if thatd make it too ios-tied but itd be utilizing something that already exists rather than having to make something from scratch i guess. Personally i think itd be super helpful 
ill link some crappy samples
I’ve used a workaround for this in Nomad. Select the view you want to markup & save it in camera views. Capture screen & mark it up outside of Nomad. Bring into nomad as a reference, changing overlay and alpha until you line everything up. Then resave camera view.
Seems like a lot of steps but can be done really quickly - best results are if you use different contrast or maybe wireframe to take the snapshot, this way you can control the difference between the two.
Another method I use is to sketch changes into a layer on the object. Can then make your changes and delete the linework layer when done. Advantage with this second method is you can rotate the camera and work on the model normally whilst still seeing the markup.
yeah, exporting and editing it externally then importing is what i do but i rarely do it since its that extra hassle of ging back and forth every time you want to edit something, as for the second method its nice for some things but that ties it to geometry-surface and is tied to substraction&subtool. also neither method could do quick screen annotations if you were to do livestream or so for instance. if you want to try you could put a plane in front with lowered opacity and doodle on that, but that takes up memory since you need to bump up the res for drawing cant grab anything behind it while its visible (i think?) and the line’s kinda fat unless you zoom in but its another approach that can be done.