What I found out for Avis is that if you book with their .fr website then you can specify T1 pick up and drop off, but I had used .com. Unfortunately, I was in a hurry after a sweaty jog to pick up the car in T2 so did not ask about this and was probably not aware at that point and just hopped in the car thanks to Preferred rental. I was back at the airport picking up and dropping off a few times during my stay in Nice, so talked to one of the people at the Avis T1 desk. I got a vague answer in the French way, but the person said nothing about a code to enter the parking area at T1.

So upon return, I drove up to the gate for this outdoor parking area, which is right opposite the terminal maybe 50 meters from the check-in at the elevated level, so there is a small ramp up after the gate. I pressed the intercom and had a discussion with the guy, who told me that I needed a code from the car rental company and that I needed to go to T2 if I didn't have one. I persisted and he suddenly said in English, "Hey man, ok, but they are going to charge you like 20 bucks [not euros]". That would have definitely been worth it but I did not get a charge. In this area there is like a dozen spaces each for all the major companies and very confident that Europcar is included. Then I dropped the keys in the box at the Avis counter in T1 and that was that.

This was in August 2023. I described how to get to this parking area above, just follow the signs for car rental as you approach T1 and you will pass through an automatic gate at some point during your approach. It's worth a try in any case.

I checked avis.fr and they still allow selecting T1 or T2. There is no option on .com.