I think the circuit here might work (I Googled opamp voltage clamp
):
http://www.teamfc3s.org/info/articles/fcd/diy_fcd.htm
Put a transistor+base resistor or mosfet between the bottom of the 10K pot and ground. Make Vref +5V. With the transistor's input low, the opamp's non-inverting input is at +5V and will limit only above +5v. Higher, if Vref is higher. With the transistor's input high, the opamp's non-onverting input is at +1V (set by 10K pot) and will limit above +1v.
OK, the input logic is opposite of what Tom wanted, but another transistor and resistor would invert that.
Ken