I am not what I'd consider an expert. Still, aluminum comes in various alloys and tempers. If the parts are cast, I doubt you would cause any harm to them, with powder coating.
Your modern aluminums are all alloys or mixtures. Pure aluminum will corode, rather quickly. Basically, in the machine shop we used 6160 and another type, that was like cast aluminum. The 6160 is hardened in a relative term. It is heat treated to suspend elements in its solution. Over time the solution goes back to its natural form. Heat accelerates this process.
As to cast aluminum, there are different alloys to achieve different results. Basically, I am somewhat familar with sand cast aluminum and die cast. Die cast is what you see a lot of. It is ecomonical on parts that will see high production numbers. They also have good dimensional toleranceing. Sand cast aluminum isn't as accurate, but is stronger that die cast.
Since I think you are going to have your valve covers powder coasted, I would think up to 400 F you'd be OK, possible more.
Tom