I now realise my comment was not plausible, due to the fact that the water pressure was to reach 200 PSI, it would require an air inlet pressure equal to or greater than that of the water. The pipe in those pictures would only be at atmospheric pressure.
I also thought the pipe may be for routing the water whilst the hopper was being filled and your suggestion about a dual purpose for dust control also seems credible.
As the Olympic had its pipe below the ash ejector, and the Ceramic had it above the ash ejector, would it have been possible that on the Britannic they may have relocated it as well.
I wonder if anyone has photos or drawings of the system in the Britannic seeing that it was constructed after the Ceramic.