how much space inside? Could a bar / plate fit up the back inside
It clamps tight like a snow ski boot, so my guess is that it would probably be unpleasant with a bar and bolt/rivet heads, but I don’t know for sure as there is a material boot liner you wear as well. i also had concerns about it interfering with the release mech, but also my delicate fleshy heal being a thin material liners width away from a metal bar that may or may not do something unexpected, and it might just be unbearably uncomfortable anyway. Hense, looking at the other option of riveting elsewhere (or as T1000 suggests, Bolting).
I really didn’t want to fork out for a new boot, but probably agree that a couple of bolts thru plastic probably aren’t going to hold.