Environmental mapping is controlled by how light or dark each part of the environmental map is. To alter the overall effect I'd simply make the whole map lighter or darker.
As there is now no way to control the diffuse and ambient lighting on a skin it is going to be a question of balancing the other effects on the various maps so they are not overpowered by the game's global settings for diffuse and ambient lighting. Until these settings are finalised there's not much point trying to do this though.
My bare metal skins have the baked-in environmental effects that Cit describes, as well as baked-in bump effects, specular effects and so on. For those skins that include maps to control these effects I haven't turned the baked-in stuff off, but I have turned it down. My feeling is that not every player will have bump-mapping etc turned on, so for them a skin with no baked-in effects will look bad. OTOH the baked-in effects will conflict with the mapped stuff when the plane is at some angles, so I just set it so the mapped stuff is more powerful and dominates.