"I have recently started increasing the replenishment rate by pouring off a few hundred ml of PO at the end of a big printing session. I have done this because I think that it may help keep the highlights clean and also help eliminate any possible staining that might occur. This is not based on direct experiential data, but is simply a hunch based on some anecdotal evidence I have."
What you describe is the way I have always worked with kallitype, and with sodium citrate developers with palladium. If you don't replenish on a regular basis with kallithype (about 100ml for every 300-400 square inches of print developed) it becomes increasingly difficult to clear the print. Perhaps it is also true with palladuim, though on a lesser scale.
But how do you deal with the accumulation of metal salts in the developer? I have heard that they fall to the bottom of potassium oxalate developers, but I have a liter bottle of a 30% solution of potassium oxalate that I have used with palladium for several months and I don't see any metal sediment on the bottom??