Technically the mesh-change-upon-equip should already be useable, if not at the moment available to players (the code is there) - what needs to be done for the heavy armor to be effective is the armor skill itself however. Right now the torso piece is the only changeable armor mesh, but I just looked and it seems the Ylian folder within characters.zip (in the art folder) has the plate textures for arms and legs and everything as well. So if you want to see what the entire suit looks like, simply rename the textures so they are the Ylian's default (for example ylianm_arm_plate.dds would have to become ylianm_arm.dds). Remember to backup your original though so you can restore it later.