Are you using Draw Texture or GUI Textures?
GUI Textures are an older GUI system in Unity, but still quite powerful:
https://hutonggames.fogbugz.com/default.asp?W361GUI Textures give you a lot of control over layering - just play with the z coordinate. You can build the whole UI in the Unity editor, then use PlayMaker to manipulate the GameObjects (Enable, Transform etc.). Check out PlaymakerSamples\TestLab\GUIElement for examples.
Draw Texture uses Unity's OnGUI commands (GUI and GUI Layout Actions). Since actions are executed in order, you can just re-order the actions to change their layering. But it gets tricky when GUI Actions are spread across FSMs since you have no way to control the execution order of each FSM (or any MonoBehavior/Script in Unity for that matter).
You can also just use textured planes in the scene, parented to the camera. Then use Playmaker actions to manipulate them. Hint: Build a hierarchy where related gui controls are parented to a root object so you can easily show/hide, move etc. the whole gui.
Lots of ways to do this in Unity/PlayMaker! I'd play with a few methods, and pick the workflow you like best...