I think the only way to make something like that work would be to use something like FSUIPC or similar to make the controller axis report more than the normal 0-100% range. Thrust reverse in the sim is done by telling the core code that the throttle is below 0%. This is most commonly done by holding the Reduce Throttle key - as in F2 back in the FS9 days. So you'd have the axis somehow report a range from -25% through 100%, with 0% calibrated to match the idle detent. While theoretically possible, I suspect this would be a tedious undertaking to get right. Asobo could also add the option to report below zero in the sensitivities menu, but I suspect that would cause even more confusion there than is already present.
What I have on my setup is a button by my throttle thumb bound to "Toggle Throttle Reverse Thrust." On landing, I press that button, and then I as move my physical throttle forward, the in-sim virtual throttle moves into the reverse range. It may not be quite "real world," but it gets the job done without having to let go of the control.