A search for "See's" "ash ejector" causes Google.ca’s preview of “ENGINEERS AND ENGINEERING IN THE ROYAL NAVY” at onlinelibrary.wiley.com/doi/10.1111/j.1559-3584.1914...x/pdf to show page 583 from the Journal of the American Society for Naval Engineers, Volume 26, Issue 2, May 1914. It says that “The type, known as the See’s ash ejector, was found to work most successfully, but it left a trail of ashes in the wake of the ship, and to obviate that a later form of ejector is used which the ashes and water are discharged through a pipe in the ship’s bottom.” (the later form was Stone’s). I think this shows that there were some improvements being sought in the system so our pipe may still be to do with dust control. CC Pounder’s Machinery and Pipe Arrangement pg 332 mentions that fine ash blowing back on deck was a disadvantage to See’s.
Now for the latest posts, as you say Ralph an air vent on the funnel overflow loops breaks any vacuum, that prevents minor amounts of overflowing water from starting a syphon effect and emptying the water tank. And a WC vent is to do with the water trap, when waves hit the side of the ship they push the air in the discharge up and without a vent, up goes the trap water into the WC. Conversely when the wave goes down it could suck the water out. At the ash ejector however I don’t think there is a water trap, so using our pipe as an air intake seems unlikely.
Another book showing See’s, Steven, is "Verbal" Notes and Sketches, JWM Sothern, 1916 http://www.archive.org/details/verbalnotessketc00soth Page 148. It too shows the anti clogging air vent you pointed out. It also mentions having the water supply pump brought up to pressure first. As that is 200 psi that means that closing the ejector’s lever operated valve when you are done could give the piping quite a water hammer shock. With our pipe appearing to connect to an older, separately mounted control valve an alternative use for it could be to accept a diversion of the flow when the ejector is stopped or cycled. After the fireman is done he could shut the supply off softly with an ordinary valve. Verbal’s figure, item P, includes the phrase escape valve which adds to my suspicions here even although this one just appears to dump into the bilge. The usage could even be simultaneous with dust control.
You don’t suppose the Olympic had the discharge pipe as a relief and then someone said raise it a bit and it’ll do dust control too ???