The idea struck my mind while I was looking at this typical application for the PCF8563.
This RTC requires typically 250nA (interface inactive, like in circuit in power-down), but a general-purpose diode like the 1N4148 already leaks 10% of that. The gate reverse current of a JFET is only a fraction of that; max 1nA for the MMBF4391.
The whole circuit is very low voltage and dito power. Are there reasons not to use a JFET as a blocking diode here?