I just learned a little more information about this topic; looks like Sword of Moonlight completely ignores all material settings (diffuse, emission, specular, alpha etc) when making a map Piece model and instead uses a single standardized material setting for all map Pieces. The significance being, that when making a map Piece, changing material settings in Metasequoia will have no effect.
For Object models though, Metasequoia's "diffuse" material setting changes the overall color brightness. So if you set "diffuse" to 0.5, you'll end up with the same effect as if you darkened the material's "color" setting by half (ie RGB= 128). 
For Object models though, Metasequoia's "diffuse" material setting changes the overall color brightness. So if you set "diffuse" to 0.5, you'll end up with the same effect as if you darkened the material's "color" setting by half (ie RGB= 128).