Consider the following sentence:
νυνὶ δὲ καλῶς ποιήσεις γράψας μοι περὶ τῆς ὑμετέρας σωτηρίας καὶ περὶ ὧν ἄν σοι ὑποπίπτηι (TM 7340, sometime between 200 BCE – 1 BCE)
Now please write to me about your welfare and the things about which there might be a liability for you
This is a fantastic example of what has been call prepositional ‘pied-piping‘ in syntax. It was first introduced to the world by a brilliant linguist (I mean shockingly brilliant, Ross 1967) under the watchful eye of Noam Chomsky. In short, languages avoid unacceptable island effects by preposing certain types of constituents like wh-questions. As Ross originally had it, this explains certain puzzles in the syntax of constituent questions:
“I wonder which person you met at the party?”
#”I wonder you met at the party which person?”
Pied-piping is standard fare now in syntax, and no one would question it. However, it is a transplant from transformational syntax and is therefore like the light of a star now long dead. Yes, the light will help you find your way –– but it is still dead light. And I suspect biblical scholars now miss the original context of the term: Ross (1967) was trying to explain the structural facts of constituent dislocation and the various island effects it creates in English syntax.
Biblical scholars need to be careful that they are not simply slapping new names on old philology. Pied-piping was not why speakers chose a relative clause. We might say περὶ ὧν was pied-piped to the front of the clause because Greek disallowed preposition stranding. But what does this explain? We already know it is there––why was it chosen at all?
A better explanation is that ὧν is a non-restrictive relative clause that embeds a presupposition about known information highlighted by the author (e.g. the constituent is under focus): the writer is saying there are some things, I just don’t know which, and you should write me back about them. This is why he asks the recipient to write περὶ ὧν… Is it an example of pied-piping? Sure. But that does not explain why the author used it. Ross is a boss, but biblical scholars must beware extracting linguistic terminology from its original context and proposing it as an answer to questions it was never intended to address.