Inasmuch as reverser aero behavior can reasonably be correlated with IAS, I'd think Boeing intended the reverser T/L reduction to be keyed on IAS.
The fact that a 40kt headwind might show a 60kt IAS at taxi shouldn't bother reverser operation, should it? I'd be more concerned about tailwind ops.
But if you're looking for an official guideline, ask your Boeing rep!
