Not an A/C expert/authority at all, but I overhauled the system in my Ram and it works wonderfully (38-40° from the vents even at 100° ambient).
So I replaced every component with new, when I finally got around to hooking up the suction-discharge lines to the compressor, I set it up on the vacuum pump and pulled it down. Shut it off and about 5 minutes later it lost all vacuum. Try again and see if I can’t hear any sucking noise. Nope. Ok so I put 100lbs of shop air (supplied through a filtered setup) in, spray around, nothing I could find, let it sit for half and hour and hadn’t lost any pressure. So I pull a vacuum on it again and it held for an hour with no loss. Go run the pump for another 30 minutes or so and charged the system by weight and it works excellent.
My guess is a seal maybe was upset and the positive pressure was enough to seal it? I did that back in April and it gets used basically daily and still frosty cold.
I say that to say, I don’t think a little compressed air is an issue if nitrogen isn’t available as long as it’s quality compressed air, not visibly blowing water out or oil with the air.
Maybe not most ideal, but there are a lot of systems that have been opened and exposed to atmosphere for years or decades that are repaired, vacuumed, and charged and work fine.
Just my 2¢. Now if you didn’t/were not going to vacuum the system down before charging, I’d say don’t do it.