Tony,
I talked to some friends who run tree farms (evergreens) who use planters like the one you are considering. Basically the speed limitation has to do with how quickly you can pick up the plant and get it into the planter, and how close you want them planted. If your wife can get them into the planter fast enough, you can go faster than the 0.2mph. Also, if you can drive the tractor across the garden at idle (800rpm) without stalling, then that works out OK.
Some of the larger Yanmars have creeper gears which would be perfect for your application. Unfortunately, the YM1401 doesn't have the creeper gear.
Greg