I think it depends on how it looks. I’ve seen people walking around wearing papier mache stuff. No one has commented on it.
If they are going to make imitation leather then they would have to be sure that they it will look realistic and if someone interreacts with it, it will act in a similar way to leather.
I don’t know how easy it would be to do but maybe they could try making it from reeds and wood instead. It might be harder but it would be an interesting look.
May I ask which nation you are going for? That will also determine what they can wear. For the marchers you could wear gambeson as light armour. It wouldn’t work so well for navarr. I hope this will be some help, I think it would be a good idea if you email profound decisions about it to make sure.
(Sorry I forgot to say welcome. I keep doing that, I get caught up in trying to help and usually I do it while I’m tired. Anyway, welcome to larping and empire, if you have any more questions feel free to ask.)