No I do not think JD has some "super secret" additive, but if you look at the specs of the lubes you mentioned they are clearly not the same. Blenders will additize lube to whatever spec a company wants.
A good example of this is Amsoil, there basestock comes from several different suppliers, its the additive package that makes the oil, not the other way around.
Sorry, but you're making it sound as if Deere is some fly by night company rebadging an off the shelf lube, dumbing it down, as you put it. When in fact through my research I proved to you the Plus 50 is an exclusive blend, no different than Cat branded lube or countless other engine/machinery manufacturers.
The two lubes you mentioned are very good, along with others, but they don't boast a 500 hrs. OCI either. One Deere marine engine I have came from the factory with a 250 hrs. OCI, unless using Plus 50 then it can be extended 50%. Then they introduced Plus 50 II extenting the interval to 500hrs., double the original service interval.
Can this be done with other lubes, sure with UOA's to back you up, but from looking at the additive package this oil looks very good and can be had for a little over 11 dollars a gallon bulk, I honestly don't know how one could beat it.