I was overcomplicating things. I thought that you always had to use a State Machine to transition to different states, and while that can be true of full-body animations, if you’re using a layered blend per bone, then you need to set things up inside the Anim Graph.
So what I did was added my blend space to the anim graph and saved that as a cache, then used BlendPosesByBool to switch between using the Jetpack’s lower body animations, or the regular full body ones as the default state which becomes the lower body animation set when using the UperBody cache.