Espruino build for nRF52832 board with external low-frequency 32678 kHz crystal

Oh, so you want to add crystal to existing MDBT42Q board? And now I also noticed

I would like to use the external low-frequency 32678 kHz crystal to improve the board power consumption when used as a beacon.

How much of power you expect to save? It may not be that much and it is likely other components on board may draw more. See e.g. this https://devzone.nordicsemi.com/f/nordic-q-a/25540/is-external-crystal-neccessary-at-all-on-52832/100707#100707 so it is really not much. Was thinking you already have board with the crystal and just want to enable it so it is not sitting there wasted.

And BTW there is also the builtin DCDC converter, it could save more if used , that is however more complicated to enable as it needs more components. see https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/power.html?cp=4_2_0_17_0#topic and https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/ref_circuitry.html?cp=4_2_0_52_1#schematic_qfn48_dcdc

just googled and here https://www.programmersought.com/article/18503111047/ someone claims enabling DC/DC can save 40% and crystal 1-2%

Anyway, yes those board file changes will help freeing D0 pin, however you need to build and update also the bootloader as that is where the button is actually checked.