i'm not saying to use a rocker squat in place of a heavy squat but as a supplemental exercise that could be trained with some decent weight. for example, depending on how on'es training program is set up you could do rocker squats on your light day or as a secondary exeericse on your heavy leg day after your squats have already been done. as for load limits i wouldn't sugget using 650+ pounds on most of the boards out there but i'm sure you could build one that could handle that kind of load.
The only thing that has helped me get a heavier squat was squatting heavy, and I believe I've tried most things.
Even front squats never gave me any carry-over when shit got heavy… that said, I never put the effort in to get a 315 fs.