Being thwarted in our collective efforts, we turned to the wisdom of the Internet Movie Database, which confirmed the presence of the character in the movie, played by an actress named Genevieve O'Reilly. Ms. O'Reilly was also noted as having played "Officer Wirtz" in the latter two of the Matrix pictures. None of this helped, and we were a bit stumped. Nobody remembered an Officer Wirtz from Zion or the Matrix itself, so we figured that Mon Mothma was another one of these "If you look at the left side of the screen whilst occluding your eye to the seventh house of Jupiter, you will see her shadow for approximately 0.75 seconds during the sequence where Senator Skywalker makes her observation about thunderous applause" kinds of characters.
However, trust the Internet to bail out the semi-obssessive fanboy in need of a fix. The IMDB message board for Ms. O'Reilly saved the day, telling us two things:
1. The scenes with Senator Mothma (no relation to Senator Mothra, from the island planet of Toho) were cut.
2. Officer Wirtz was in the Zion scenes, with spoken dialogue in Revolutions, asking "What are they doing?" in response to an action taken by the sentinel horde.
A quote, relayed second hand or so, from Rick McCallum (Mr. "George Doesn't Think the Technology's There So That's Why We're Still With VHS" Producer dude in the blue denim shirt a couple of years back) suggests that there'll be deleted scenes on the forthcoming DVD release. I certainly hope so; it seems a waste for Ms. O'Reilly's character to not make the screen.
At any rate, now that that mystery's solved, off to less profitable things. I'm just glad I wasn't somehow missing a giant chunk of the film. See here for one more-or-less authoritative fan source of information WRT the character. The Star Wars website offers The O'Reilly Factor and the 'Databank' entry for Senator Mothma.
I'd vote for her.
1 We do not recognize the new regime in place. So far as this publication is concerned, Kenner is still making the Star Wars toy line. Hasbro, on the other hand, manufactures the G.I. Joe and Transformers toy lines. Your correspondent is, if nothing else, not about to change terminology simply because twenty-odd years of corporate mergers have intervened.