There is a bug that allow you to use the mount without having the staff equipped. Make a macro like this:
/equip Dragonwrath, Tarecgosa's Rest (optional)
/use Dragonwrath, Tarecgosa's Rest
/equip [button:2] "your original wep`s name"
Then if you want to mount up: left-click the macro then fast right-click it. Simple as that :)