As a bonus mentioned yesterday, I tested if gouache varnish (thinned with white spirit) will work on minis and surprisingly it does. It definitely is there as I can feel the difference under fingers but aside from very subtle matte effect on surface, it barely change surface it was applied to. Maybe if I would use it in less thinned form (in my case it was 50-50) that would change but producer (Talens) claims it can only occasionally darken some shades but should otherwise have no other effect other than forming protective layer. So, this seems like option too.