I've been working on this for several hours. There is no information available online about who were the sergeants-at-arms for the 1912 convention. If the information still exists anywhere it would be at the Atlantic City Free Public Library. Their archives are not online, nor are the indices. However, people are welcome to visit them and research in person. Here is the contact information:
Atlantic City Free Public Library
1 North Tennessee Avenue
Atlantic City, NJ 08401(###) ###-####/p>
Please let me know if you have questions.
I expect that the archives will have local newspaper clippings of the event, which might mention day-to-day proceedings, including names of sergeants-at-arms and other officers. I know they do have newspapers back to the late 1800s up until today, so that would likely be the best resource. They may even have records separated of ALL political conventions held within the city (it would be HUGE, since Atlantic City has always been a popular spot). However, getting to the information will be the hard part. Atlantic City might be a great destination vacation if that fits within your plans for the next year :-)