I am going to answer my own question. There is a simple "agitation kit" available for sprayers of this size. I got it for $20 from Northern Tool and Equipment. It consists mostly of stock parts you could pick up at the hardware store, except for the plastic tube that runs through a hole you drill in the top of the tank and that connects to a Y fitting with controls to regulate how much goes to the sprayer wand and how much to the agitator. However, to call this am agitation kit is a gross exaggeration. the spray just comes out of the open end of the tube about 4" from the bottom of the spray tank, under no real pressure because there is no nozzle. No agitation occurs. So I remedied that with a couple of compression fittings from the hardware store, a short piece of copper tubing to go between them (to get the agitation nozzle down to the bottom of the tank), and then screwed on a standard nozzle from a garden sprayer. (the bottom compression fitting is a right angle with one end threaded.) Now the spray comes out of the tube with enough pressure to actually agitate.
You might wonder why not have the agitation nozzle enter the tank at the bottom. If you do that you will need a good way to seal off the tube where it exits the wall of the tank where you drilled a hole. With the hole in the top of the tank, leakage is not an issue.
This would work better the more pressure you have from your sprayer pump. I shut the bypass valve almost completely off and then adjust the two valves that came with the agitation kit to get the right balance of pressure and flow in the sprayer wand versus pressure in the agitator line.
I would add a picture of my setup but I don't see a way to do that.
Turkey Creek Orchard
Solon, Iowa (zone 5A)