Since no one that has specific chemical knowledge has replied I will give you my take on it.
I would avoid Vinyl, PVC, Neoprene, Rubber and most foamed type of plastics. I believe it is the plasticizers which make the materials soft is what eats into Nitro finishes. It doesn't stop leaching out of the plastics and once it gets into a nitro finish it is a bit like a flesh eating bacteria that just keeps penetrating the finish. I have repaired a few guitars that have had stand marks on them and I always sand beyond where the marks are to remove any of the softened lacquer before spot finishing.
If you want to be sure your product does not effect any finish then I would suggest either leather or cotton. I have a guitar stand that had rubber tubing on it that I pulled off when I first bought it and replaced it with medical grade neoprene tubing but it still left marks on one of my guitars so I usually lay a towel over the stand first before I put my guitar on it for any length of time. I will put them on the stand without a towel if I know it is only going to be for a few minutes.
I don't really know what the specific plasticizers are that will eat into a finish since I am not a chemist but I would personally avoid any plastic or rubber if I were designing a stand and would probably go with a soft leather or something like Microfiber.
That is my 2c
Cheers,
Bob