[Mathias]

I haven't tried this out yet, so I don't know if I missed anything.

Near the bottom of the editor entry is a section for user tags. Go in there for your feat and define one, for example, ArmorOptim.

Now, in a script on your feat, running before Mithril reduces the armor category (First/10500), use the same foreach you're using above to find all the heavy armors, and then execute the following:

perform eachpick.assign[User.ArmorOptim]

You've now tagged all the heavy armors with something that won't be altered when mithril operates.

For the later search, look for that tag alongside everything else:

 
foreach pick in hero from BaseArmor where "mClass.Heavy | User.ArmorOptim"

Now, you'll find all the armors that are heavy armor at this point, or were heavy armor before mithril was applied.