I've tried without success to use a VFDs current sense as
an indication of overload on a conveyor. The difference
between normal load and gross overload, short of stalling
the motor, was not large enough to sense reliably.

I have experienced the same. I look at the motor current
and then load the machine and I only see very small changes.
Therefore I question the VFD current display.

I have had my mill kick out on overload when drilling (high load)
at very low freq(10Hz). I think it has seriouly derated the motor
at that speed though.

chuck