>I've used series (REF-02) voltage references for precision low current
>regulators in the past.  You might look into replacing the zener with a
>micropower shunt regulator (bandgap ref) that will regulate much better at
>low currents (10 uA).

The above idea is pretty good, and may be better than mine, which was to
investigate the micropower chips used in cellphones. If I remember correctly
Maxim make micropower regulators for this use, but I doubt they will have the
input voltage requirement you want.