Diode behavior when reverse biased with small voltage
One of the best simple models I've seen is here: 

This shows that the reverse current quickly enters a predictable saturation region with mainly the temperature effects to be aware of. However there is a much higher resistive leakage current that impacts the total reverse current.

This diagram shows both Is (the saturation reverse current) and Il (the resistive surface and bulk leakage).



It's worth noting that the resistive leakage value can be permanently changed (increased) by overheating the device during soldering. This makes it very difficult to predict total reverse current in any simplified model. Resistive reverse leakage may be in the 10 - 1000 MOhms range.

Note: If your goal is to provide BBU for something low current like an RTC chip with a CR2302 battery, then you don't need to use diodes at all. You can use a transmission gate to switch from one supply to the other. For example the Philips 74LVC1G3157 is capable of up to 12mA with off state leakage below 1 uA maximum.