More info on PINIO can be found @ Betaflight GitHUB: https://github.com/betaflight/betaflight/wiki/Pinio-and-PinioBox .
I guess you need to find the free output (e.g. Motor 5 or S5), then find the MCU pin name for that resource with RESOURCE MOTOR 5 command (RESOURCE LIST command list all the resources). Ten remap this MCU to the PINIO resource.

Where can I get more information on the following commands? I try to understand them as don’t seem to to be able to use them verbatim in iNAV on a Matek 405 board… I want to be able to switch the bluetooth boar on and off.

set pinio_config = 129,1,1,1
set pinio_box = 0,255,255,255
resource MOTOR 5 none
resource PINIO 1 A15

ref: https://www.speedybee.com/product_images/download/manual/SB-BUA-manual-EN.pdf