Therefore, if the current limiter is connected with a +5-V supply, the load will get ~3.4 V, which is unacceptable in low-voltage circuits. ... a Zener-diode acting as a regulator.

I'm told (from the Internet) that 0.9V is about as low as you want to take a NiMH battery without worrying about damage, so I guess I'm looking for something trivial to build that cuts off at 1.8V.

Specialized Circuit Drives 150-V Piezoelectric Motor Using Low-Voltage Op Amp Oct. 23, 2013 Use a basic op amp and discrete components to drive a piezoelectric motor and provide circuit protection.