Switching device for ultra-low power sub-circuit
Because you've done some adequate research into a solution and have put together a half decent requirement I'm making an answer...

The ADG819 2:1 multiplexer operates from 1.8 volts to 5.5 volts and has a maximum D-S leakage current of +/- 10 nA across the full -40 C to +125 C temperature range. On-resistance is 0.8 ohms maximum and this would result in a volt drop of 16 mV at 20 mA current through the channel.

If you want more switching options the ADG1604 (4 channel multiplexer) is also pretty useful. On resistance is 1.6 ohms and D-S leakage is 16 nA max.