My first question: Are there explicitly stated rules banning the practice of using a deholdered slab insert in the registry?
If no, and it's still a question of "ethics", the next questions that come to my mind are: Who will be harmed? And to what extent?
My suspicion is, since the registry is meaningless to me (and almost certainly most collectors), and nobody is really going to be harmed, then I'm not concerned that there's an ethical question here at all.
(Of course, since I do not participate in the registry and am unaware of the rules, my answer is based on a lot of assumptions.)