This diode is a bit expensive and too large for your project, but for experimental purposes it seems like the cat's meow as a blocking diode when collecting currents at tiny voltages: http://www.ti.com/lit/ds/symlink/sm74611.pdf Just 26mv forward voltage drop at an 8a current, and just 0.3ua reverse leakage current at a voltage of 28v. Obviously, those numbers would be far less for the currents and voltages that we're dealing with. It seems to be very nearly an ideal diode, or at least the closest I've ever seen to that.

Right you are. Live and learn (even at 81), and I even learned a new word -'vias.' Thanks.