JFET as blocking diode?

Jfets can be used as diodes. TI has an app note on this where a diode connected JFET is used to protect an op amp input from overvoltage. Leakage current was in the sub 100 femtoampere range (http://www.ti.com/lit/an/sboa058/sboa058.pdf). See also: Current Sources and Voltage References: A Design Reference for Electronics Engineer p180-183

The 2N4117A referred to is no longer available, but the MMBF4117 is a good replacement, and has the same guaranteed maximum leakage of 10pA at room temperature and a reverse bias of 20V. Note that the leakage current is less at lower voltages, so since Vdd is at most 5.5V for the PCF8563, the leakage current is even less.

For best results connect the source and drain together as one terminal of your diode and the gate as the other.

A note regarding temperature and this applies to all PN junctions, diodes (including low-leakage diodes), JFETs, BJTs MOSFETs (etc):

The reverse-biased leakage of a P-N junction has a strong positive temperature coefficient, approximately doubling for each 10°C increment in temperature. This exponential increase racks up quickly as shown in the normalized graph of figure 1. At 125°C leakage climbs to approximately 1000-times the room temperature value.

source:https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/11/14/temperature-effects-on-input-bias-current-plus-a-random-quiz

1N4148 is a general-purpose, legacy diode that you shouldn't use if you're looking for some level of performance.

From a very cursory glance at Digi-Key, the BAS116 (in stock, 0.25/10, 0.035/reel) has a typical ~400 pA, max 5 nA leakage at 75 V, 25 °C.

Not sure if it's an actual "series", but BASx16 seems to go to BAS416, a 75 V, 200 mA diode with 3 pA typical, 5 nA max reverse leakage @ 75 V, 25 °C. In-stock at Mouser.

The JFET you linked (MMBF4391) doesn't shut off until -10 V (though the MMBF4393 in that datasheet shuts off at -3 V), and the quoted 1 nA leakage is at -12 V (for either -1 or -3), which you can obtain? "Shutoff" is defined as where the JFET conducts 10 nA.

The diode's leakage is specified at 75 V, so if you're running it at 3-5 V the leakage will be much lower.