Hello redbeard,
Yes -- in Hauptwerk v5+ "All perspectives: swell boxes mod: amplitude (pct)" is the voicing adjustment you would want to use (which is the equivalent of the v4 "Swell boxes: amplitude (dB" voicing adjustment). To make the swell box attenuate amplitude more strongly, make sure that you have the desired Swell rank(s) selected on the voicing screen, select that voicing adjustment, and then use (e.g.) the 'master' slider to turn its value up to a value greater than 100%. For example, if you turn it up fully to 1000% you should find that the Swell ranks become very quiet when the swell box closes.
(I don't have the dry Skinner sample set installed at the moment, but for good measure I just verified that it works properly for me in St. Anne's.)