Hi Everyone,
I've tested the circuit and thought I'd post the results.
The mosfet solution only kind of works, and not as well as the Schottky. Using the comparator to switch the mosfet on/off works fine, but there appears to be a few millivolts leaking through (150mV), though at worst it trickles through the mosfets 100k+ resistance so any effect on the battery would be negligible I think. Worse though, when the battery power is run through the mosfet with Drain attached to ground (this is opposite the normal switching use) then there is a voltage drop of almost a .6V, worse than the Schottky. This was unexpected, and I think is caused by the diode that connects Source to Drain. I thought that once the Mosfet closed the battery would bypass the diode and there would be no voltage drop, but that does not appear to be the case in my tests. At any rate, I've found that in practice the Schottky will work fine for my application, which is good because it's simpler and cheaper. So I won't be looking into this much more, but I wonder if anyone else will. Thanks again for your help, especially hero999.