That was a consideration early on, but in practice it won't work. The diode drop can't be tolerated unfortunately. The cap was sized such that with the lowest Vin, worst case SD card voltage requirements, worst case SD card write time, worst case temperature, as well as long term aging that we will always have enough time to complete the write, close the file, and unmount the SD card. By putting a diode in place, we'd either need a larger super cap (already at 47mF) or we'd need to use a FET's to short out the diode....
Hmmm... using a FET to short the diode might actually work. We know pretty doggone quick when power is lost. By the time we figure it out, the supercap voltage shouldn't have dropped very much. We could drive the FET in an interrupt. I'm off the project for the time being, but will be back on it in a few weeks. I may look into that solutions. Thanks!