Overlay a pose over idle animation in mechanim

I imported a series of "static" one frame skeletal poses in a character fbx file. I can then get these poses to override the main idle animation, but I would really like to make it so that each pose has a degree of idle movement.

Each "pose" was imported as a single keyframe within the timeline of the fbx character import. I dragged/duplicated copies of each pose animation into mechanim as a separate layer above the base idle layer. tweaking options such as additive/overide weight etc, seems to not have the desired result. Is there a simple way just to set each pose and preserve the idle animation?

Or should I be thinking of using code to make each pose/procedurally animate with small movements?