(Un?)fortunately as aircraft can have a wide range of complexity a lot is left to the aircraft developers, and while there are a lot of common things to bind to there will ultimately be differences between aircraft.
The bindings bind key-value pair properties in the property tree, which kind of is the backbone in FlightGear. You will need to figure out which property to bind to. Likely this will be somewhere in the /controls/engine/ subtree. To find the actual property to bind too use the property browser that is built in into FlightGear and toggle the reversers from other controls to see which property to bind to.
Some bindings can be configured from the joystick configuration dialog, other will need that you open the joystick configuration file in a text editor. The configuration file is an XML file and is located in a subdirectory to ../data/Input/Joystick/ in your FlightGear installation.
Since the normal binding does not seem to work, you might, If you are lucky, find the binding in the
aircraft-set.xml file in the root of that aircraft's directory. If you are really unlucky however, the reversers might be controlled by embedded scripts that could be in the
aircraft-set.xml file or some of the XML files in ../Aircraft/
Aircraft/Models that control the animations.
These FlightGear wiki articles will likely be of help: