Mark,
Something to think about when wondering why you'd want the nitrous relative to throttle angle. In real world application when you're racing a high horsepower car, you're fighting for traction and feathering the throttle throughout the run to get said traction. Think of this mode as emulating adding more horsepower through a bigger motor in lieu of an add on. You still need to modulated the throttle to get the best traction.
If you had this programmed to come on at WOT and linear in relation to RPMs from 4000-7000 RPMS as soon as you hit that range in 1st gear you'd be up in smoke and then have to back off and try again, and it would be the same way until you hit 3rd gear where you wouldn't be overpowering the tires. To me it makes much more sense to go with throttle angle for real world application of horsepower. Remember, there are cars out there that can break the tires loose in 2-3 gears at WOT without nitrous all ready added.
Rick