This question is triggered by another question about wheatear the "ad" is prepositional or purpose.
In theory, we should see examples where something like this happens:
Discipuli Marcum ad epistulam mittendam ad magistrum miserunt.
In which we have a prepositional phrase (ad magistrum) inside purpose clause that is initiated by ad.
Do we have such examples?
(As noted in the comments the previous example: Iulia epistulam ad mittendam/mittendum ad regem Marco tradit is problematic, though I think that if one uses the gerund (mittendum) is grammatical and theoretically possible.)