Put the MDBT42Q in absolute low power mode

Is there a way to activate this mode using your programming interface ?

There's nothing exposed directly, no, but I guess it may be possible to force it just by poking registers (normally you'd send the request through the softdevice).

The issue is that if you want to turn the system off you'd also have to configure ways to turn it back on, and that could potentially get quite complex. Once you're at the 5uA level even things like capacitor choice is going to have a big effect on power usage, so I figured it wasn't worth that effort.

However, if you wanted to add something in a custom firmware you could look at what we do for Bangle.js where it is implemented (https://github.com/espruino/Espruino/blob/b01e9d814a748349a05bce9666b6e18d035dc850/libs/banglejs/jswrap_bangle.c#L4564) - it's literally just a call to sd_power_system_off but you've got to be careful to disable anything that might be using powerbeforehand.

I'm not against adding an NRF.off function but I think if it's just a call to sd_power_system_off you're going to have to be super careful with it to actually get the lowest power consumption.