sorry I missed this. You could do this by editing the line
ExecStart=/usr/bin/stdbuf -oL -eL /usr/local/bin/bela-cape-btn --pin 60 --hold /opt/Bela/bela_button_hold.sh --delay 10 --monitor-click 0 --verbose --hold-press-timeout-ms 2000
in /lib/systemd/system/bela_button.service
. It currently uses pin 60 (P9.27). You'd need to set a different pin, the one connected to the button of our choice.
You could also ignore the above and (assuming the polarity of the signal is correct, which I should double check) route the relevant button pin to P9.27 via a an external wire soldered to the button pin and to the back of the BeagleBone.