Hey, welcome.
As for brass, it’s a bit hard to say. Generally copper reflects the laser very well [this is why the mirrors in your Glowforge are made of copper], and brass is a copper alloy. It’s hard to find a reflectance graph for brass in the CO2 laser emission spectrum (between 9,300 and 10,600 nm) and not all brass is made with the same composition… but from what little data I was able to find it looks like the brass infrared reflectivity curve is pretty close to that of copper.
So if it were me, I’d say don’t put any mirror-polished brass under your laser for the same reasons why you shouldn’t put a copper mirror in there, it might reflect the laser back at the machine and potentially damage the head.
As for more advice about working with metals I wrote up a guide here, check out #11:
When it comes to your sublimation plates, let us know some more about them, a URL if you have it etc, maybe someone will have some ideas. I don’t know if the surface of plates like that are pretreated, which might change things.