If the dealer is stumped on a simple hydraulic return location I would be very reluctant to let them continue the work. Running a return into a tee for the loader return should not be a problem. I do not like the idea of running a return through the drain plug as where does dirt and fine metals, and moisture go that gets in the oil when it is not caught by the filter? It settles to the bottom of the sump and where is the drain plug? At the bottem of the sump. So every time you use the post driver the return oil will stir up the oil in the bottom of the sump and cause the dirt, water, and metals to be suspended in the oil and flow through system.