I defer to the 757 experts, but...
I assume the rudder and other flight controls on the 757 have triple redundancy where hydraulics are concerned. There may be limitations on the centre hydraulic system, but surely the Left and Right windmilling engines would provide hydraulic power to the rudder below 130kts?(enough to get you onto the ground safely).
The speed advisory might simply be to alert you to the fact that not all control surfaces might be working (e.g specific spoilers).