I propose you an idea: could you mark (for example in italic) the current mockup in the link combo?
Tipically is a mistake to have an object in a mockup linked to itself.
In a huge project could be helpful to recognize immediately the current mockup and therefore the next one that has a great probability to be the correct candidate for my story boards.
The curent link in the combo is marked as selected (in reverse): using the italic to display the mockup name you can preserve also this relevant feature.
It is only my “two cents” to your great products!