I found the answer to the dirt problem, and it's really easy. Kubota was extremely stupid to leave the left end of the jack assembly open so it lets dirt in.
Disassembling the jack is not hard as long as it's not seized up with dirt and rust. Once you have it apart, clean the tube and the screw carefully, and make sure the bearing is okay. Then put good grease all over the screw.
This still leaves you open to dirt, so you need to fire an enormous glob of sealant into the left (not threaded) end of the tube, sealing it off completely. When the silicone cures, put the jack together and reinstall it.
This seals crud out. It can still get on the screw when the deck is raised, but you're not going to have a lot of dirt in the air while the deck is up.