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Great circuit, thank you; a clever solution to a common problem!
As you discussed, the main limitations of the current mirror design are its quiescent current consumption and battery (reverse) voltage limit due to Q1 Vbe_max (usually -5 or -6v which limits the battery to roughly 5v6 or 6v6).
The op-amp solution to extend the voltage range is particularly nice for its very low power consumption. The LT6003 is pricey, but there are lower-cost op-amps that should do the trick too: TLV2401IDBVR (880 nA) or TLV9301IDCKR (150 uA). However, any op-amp is going to cost significantly more than the current mirror.
A solution that supports higher voltages and costs nearly the same as the current mirror is Diodes Inc. DZDH0401DW-7 (~$0.10 in qty 1K); current consumption is better than the current mirror (147uA) and it easily supports Vbat up to the FET's Vgs_max (typically 12-20V).
Thanks for this circuit, crutschow. It came in handy for a recent project in a circuit that needed to be protected against transients (using a large cap and a diode), and I couldn't afford the voltage drop characteristic of a normal diode.
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