In this post from a recent OGR discussion, I showed how I modified an MTH operating reefer for about $5 in components. The reefer can be wirelessly activated anywhere on the layout instead of just when over an operating-track-section. I'd think something like this could apply to the operating dump car. But to be clear, this is a DIY project not in everyone's comfort zone.
So to sketch out a path forward going the DIY route:
the operating-track-section applies power to the 4th and 5th rails when activated by the lever controller. The sliding shoe power pickups on the operating car receive this AC voltage and start the action. There is some kind of circuitry on the operating car to convert the AC voltage to DC voltage needed to drive the DC gearmotor that raises and lowers the bed. So some circuitry must be added to bypass the 4th and 5th rail...and instead receive power from the center and outer-rails (replace/upgrade trucks to ones with center/outer rail pickups)…and to receive commands wirelessly with some kind of radio receiver.
This additional circuitry must fit in some nooks and crannies to the extent they are available. I found the above photo of an MTH operating dump car from an MTH manual. It shows what's underneath the car's bed. I realize you don't have this car yet, but perhaps someone who does can take some better photographs from different angles and make some measurements showing how much space is available.
For example, in the reefer, the components were assembled in a manner to fit an unused space as shown above.
If there is enough interest, there is a small cadre of guys here on OGR that have been known to convert ideas like this into something close to plug-and-play. In other words, if you want to pursue this avenue, then raise the flag and see who salutes! 
Separately, there have been OGR discussions about operating-track-sections and those pesky sliding-shoe power pickups. These can mechanically interfere with some track systems. One approach has been to remove the sliding shoes and instead activate the operating car by some other means. For example you could aim a light beam upwards from the track bed using a 5-cent LED. The operating car would have a 10-cent light detector and a relay or whatever to activate the car. You must still position the car over an operating track section... but the rub is the operating track section can be any piece of track albeit modified to mount a 5-cent LED! This can make it economically feasible to have dozens of operating track sections. Again, this is just an aside.
