developed a slow transmission leak

I am impressed by your fantastic skills at getting to a good description of your questions area of concern, you have circled a vented part called a valve body selector shaft, and the screw on cap acts to vent air,not fluid, and should not be leaking. There is a good possibility that your internal components are leaking pressure forcing the expannsion of air and fluid to force itself out at too great a pressure, the only times this vent would leak fluid was if it was overfilled, or overheated. If your having no shifting problems and it has lessened its leak, just the cap can be replaced. Be sure to test the fluid level in Park when fully heated up. You may have cooling or hydraulic seal problems also, there is a good chance that it is more than just that part. I would take a good look at the fluids condition and scan the transmission control module to be sure its not a serious problem. It is older, and the 42LE had some cooling system issues. If too much fluid was put in, this was normal, so I hope the best for you. That top leads to a long hole in that long shaft, there should be no pressure leaking out all the time. A valve body(this is the assembly that this part is attached to) can fail and also cause a repair that may have made it very expensive, if you are having other shifting problems, thats why I am sure that your TCM was checked with a scan tool(computer testing device). I would unscrew this cap, seal it with a silicone sealer, and if it still leaked the next step would be to take down the oil pan on it to inspect or replace the whole valve body. It can be done without taking the whole trans out. Theres a pressure leak, a very bad one. Just like a cooling system radiator cap can leak from too much heat and pressure.