Hi Amanda!
Plantarflexion and knee hyperextension often occur together because they involve posterior translation of the tibia and are both part of the extensor synergistic pattern. By limiting plantarflexion at the ankle via a PF stop, decreased knee extension may also occur.
Excess ankle dorsiflexion would not be expected with knee hyperextension, so a dorsiflexion stop would not reduce knee hyperextension. As theorized above, a dorsiflexion stop may provide some protection against knee buckling that could occur due to the expected knee weakness/instability associated with knee hyperextension but would not change the hyperextension itself.
Hope this helps!
Chrissy