I wasn’t 100% whether or not this was an official rule, but I’m currently on an American Airlines flight as a US Airways Chairman.
The benefit for non-upgraded Executive Platinums on American flights is:
“Enjoy a complimentary snack and standard alcoholic beverage when traveling in the main cabin.”
I started to ask the flight attendant if this was a benefit extended to Chairmen on US Airways, when she proactively reached out to me and said she saw that I am a Chairman on US Airways and offered me a complimentary beverage and food item on the house… er, plane.
I thoroughly enjoyed my screwdriver and turkey sandwich. I have the row all to myself on this plane, so it is almost better than being in first-class!