FSUIPC might be the most elegant way, but I have no problems using the FSX controller settings with my X52 either:
Settings -> Keys -> Controller type Saitek X52,
category engines, event: Thrust (drop fast) as you did, iirc.
Then in column Joystick I use the Button on the thrust lever of it - in may case for my thumb button 7;
in column REPEAT the slider all the way TO THE RIGHT, that this key command issued by the throttle stick is sent to FSX quickly.
On touchdown I have the throttle lever all the way back (over the detent) to idle - and then I HOLD my assigned thumb button (7).
To get the lever back to idle in the NGX I push the the Saitek throttle forward a little bit and back on idle.
No problems.
Then again the FSUIPC method, with the possibility to 'use' the detend and have a 'real' reverse position is stylish.
sry my bad english..