I've never see a pass (cover) number printed on a reservation, except in Italy when I bought the reservation at the ticket office. Online however, Italian reservations are best bought via ÖBB (using these steps). No pass (cover) number needed.

Frecciarossa reservation purchased on the interrail portal. X is the mobile pass number.

Global Pass: Only for pass holders valid for the whole international requested route. Show your credentials on board. TICKET NUMBER SUPPORT: XXXXXX 

OK, so Trenitalia is an exception. Apparently, they also print the pass (cover) number on the reservation if you book via Interrail/Eurail. Though I have yet to see the first Trenitalia ticket inspector who checks the number.

In addition, it's cheaper to book via ÖBB (no booking fee).

But in principle, your question is valid and I don't have an answer.

Having travelled with Trenitalia, the experience is that their ticket checkers are very diligent, if the linkage between the mobile pass number and the reservation is a new security feature then presumably that could be an issue for the pass holder if the mobile pass and reservation codes mismatch or the mobile pass code is not on the reservation.