Just thought I'd add that I have just successfully gained access with my girlfriend to the Iberia lounge (operated by a third party) located near D gates at Terminal 3 in Rome Fiumicino.

My flight is VY6255, codeshare IB5961 to Orly. I booked through Iberia hoping that I'd try my luck.

The lounge dragon just kept muttering to herself in Italian. I heard Sapphire mentioned a few times (I gave her my QF card), and she scanned, and typed. We are both waiting there with bated breath wondering whether we are going to get in, and then bam, no questions asked. Lounge is pretty miserable but better than the rest of this airport.

I have a feeling you may have more luck in a non-affiliated third party lounge, as they are run for a range of different airlines and probably don't know the rules for each. Confidence is probably key.