Huh. This isn’t ringing any bells. Of course there’s a lot of precedent with doing it after the fact, with ceramic tiles and such.
A lot of the time lasering amorphous mineral surfaces ends up with melting/glassing or slagging of the surface, without any real penetration. This makes sense, because there’s not a lot of organic material to vaporize and remove, so in effect you superheat the small parts of the matrix (sandstone, soapstone), and get really strange effects.
As for clay, the moisture content adds an interesting dimension. I can’t guess what it might do, but I suspect it won’t hurt anything to try it. If no one chimes in, you should go be the first!
Here’s some previous clay posts, just by doing a simple “clay” search:
https://community.glowforge.com/search?q=clay
This one is particularly relevant: