It will probably work if can get the voltage under 15V so to not damage the SW-3B.
Maybe have a plan B ready like bringing a 8 cell AA battery holder and add the barrel connector. Worst case you buy 8 cells at the supermarket/shop near your brothers place and you are ready to activate.
Something like that:
https://www.reichelt.at/batteriehalter-fuer-8-mignonzellen-aa-halter-8xaak-p113158.html
73 Joe