Well, I think it should work to run it off your pup lines, but you will need to put a sander control in there somewhere. Normally they go in the cab, giving you independent speed control of both hydraulic motors as well as on/off for the unit itself. Given the choice Id put in permanent lines and cab controls. But, if you put the control panel on the spreader you could quick hitch it to the pup lines right at the rear. Only drawback is that you would lose the ability to control the sander on the fly. Then you could start/stop it with the pup valve you already have. Is the pup valve detented so it will stay on without having to hold it? You will have to add a return line for the spreader, again, no big deal. As long as the driveshaft to the pump is good and tight I cant see where it would hurt it at all to have it running for a half
hour at a time. Pump is probably at least 15 g.p.m., which should handle the V-box ok. The control box should probably be under some sort of shield, Im not sure how well they would stand up to being out in the weather. Keep in mind that when you first start the spreader, with cold oil, it will run slower than once it has warmed up and you will need to change speed settings accordingly. It comes back to whether you want to get out and run around the truck to adjust chain and spinner speeds. The speed control has four ports, pressure in, two out to the motors and one for return/excess flow. The return needs to be plumbed into your return line from the spreader motors.
Im sure there are electric spreader controls which would let you control from the cab with only wiring needed back to the actual valve assembly but Im also pretty sure they would be pricey.
I could probably plumb it easier than I can describe it. Too bad youre so far away.