Is there a way to lock a graphic aspect to prevent editing on a per aspect basis, similar to 'reserve' function for
I'm afraid no, 800xA Security Definitions work on object level.
I haven't tested, but if you were to create a new aspect category of Graphic Display PG2 in Aspect System Structure/Graphics/PG2/... you may perhaps be able to set role security to require higher or different role membership to enable modifications on the display.
This method have drawbacks since you can not convert already created aspects into the new category; you must open cat1 display, select all, then create & open cat2 display and paste + save there. But if number of cat2 displays are small this might be a way forward to allow you to create a number of graphics that have require a different role than the default application engineer role to edit. However, once again, I haven't tested it.
I like your request tough, and would like to see it in some future version of 800xA; so don't be afraid to speak to your regional ABB sales representative and ask him to forward this to Product Management for evalation.