Is there diode with both low forward voltage(<0.2V) and low reverse current(<0.2uA)?
You could consider using a low leakage p channel MOSFET as the "diode" such as the FDN304P from Fairchild. It exhibits an on-resistance sub 100 milli ohms when on and a leakage current of no more than 100 nA when off. You will need to control the gate to make this work so I'm assuming that you have the spare IO lines to do so.