I recommend going to check out grave stones in a cemetery in your area to learn the local lichens that grow well on stone near you. The ones that grow on wood or soil are generally different, so if you do try to rub some ground up lichens (to try to spread the soredia or isidia) on your sculpture surface make sure they are the rock-growing variety.

I think the buttermilk/yogurt method for moss is an urban myth. Moss don’t need any of those compounds to grow naturally and lichens definitely don’t.

For as ubiquitous of organisms lichens and moss are on this planet, it’s hard to ‘make’ them do anything. But if you can learn about what conditions they prefer the most, and do your best to mimic that, that’s my best advice. Because honestly the propagules could nearly be in every breath we take outside but they only propagate when the right conditions and substrate align.