I don't think there would be any legal obstacle to hybrid vaccination. Big health systems, public health depts etc will follow the CDC's recommendations. Individual docs and perhaps some university depts etc will make their own decisions. I could certainly see young women who survived a J and J getting something else for the second dose.
That HR dept jumped the gun a little I believe--the FDA still has to meet and decide to accept the panel's recommendation and then the CDC has to make its recommendation, which Walensky then has to approve. Pretty sure it's a done deal though.