The “rubber chicken circuit” is the lecture circuit, where banquets are held that serve frequently unappetizing food. “The rubber chicken at hotel banquets” was cited in print in 1930; “the ‘banquet circuit’, tearing through the rubber chicken and the hip steaks” was cited in print in January 1941. “The ‘rubber chicken, canned peas, and rubber lung’ circuit because of its combined menu of food and fiery oratory” was cited in June 1941.
   
The term “rubber chicken circuit” has been cited in print since at least December 1941. “Rubber chicken loop” is less frequently used, but has been cited in print since at least February 1941.
 
   
Wikipedia: Rubber chicken
A rubber chicken is a prop used in comedy. The phrase is also used as a description for food served at speeches, conventions, and other large meetings, and as a metaphor for speechmaking.
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Food and speechmaking
The term “rubber chicken” is used disparagingly to describe the food served at political or corporate events, weddings, and other gatherings where there are a large number of guests who require serving in a short timeframe. Often, pre-cooked chicken is held at serving temperature for some time and then dressed with a sauce as it is served. Consequently the meat may be tough or “rubbery.” Someone who “travels the rubber chicken circuit” is said to do so by attending or making speeches at many such gatherings, often as part of political campaigning.
 
(Oxford English Dictionary)
rubber chicken  n. slang (chiefly N. Amer.) attrib., designating or relating to a speaking event (esp. for political or fund-raising purposes) at which poorly cooked food is typically served; esp. in rubber chicken circuit.
1941   Daily Messenger (N.Y.) 16 Dec. 11/2   Popular story on the rubber chicken circuit tells how Bernie Bierman discovered one night that the hotel where his athletes were staying was on fire.
1959   Maclean’s Mag. 23 May 1/1   Next year’s rubber-chicken circuit is being sewed up by three Toronto women with a public-speaking agency called Canadian Celebrity Bureau.
1979   N.Y. Mag. 3 Dec. 44/2   Most of these things are just routine, rubber-chicken events.
 
30 July 1930, Seattle (WA) Daily Times,  “Telling the World: Byrd Gets Ready Again” by Neal O’Hara, pg. 6, col. 2:
“First of all, I try to wear earlaps, so I won’t hear the preliminary oratory before I speak, but sometimes it is hard to adjust them. Then I take along twenty cases of pemmican to gnaw on in preference to the rubber chicken at hotel banquets, and of course I have forty-eight tanks of ice-water or lecture juice, as it is called.”
 
13 November 1936, Omaha (NE) World-Herald,  “Pile of Cash to Braddock” by Jack Miley, pg. 31, col. 6:
All Jim has to show for 17 months of the championship is a wide acquaintance from tank town mayors and a bum boiler from packing away tons of rubber chicken at countless dinners in his honor.
 
2 December 1936, Omaha (NE) Evening World-Herald, “Braddock Is Some Talker,” pg. 20, col. 4:
“We had to do a lot of talking to keep from eating that rubber chicken at those banquets,” Gould explained.
 
15 January 1941, Greensboro (NC) Daily News, “McCarthy Looking For Another Pennant” (AP), pg. 10, col. 6:
Ordinarily at this time Joe’s eyes have a slightly glazed look and his breath comes in gasps after having spent a couple of months on the “banquet circuit”, tearing through the rubber chicken and the hip steaks.
   
27 February 1941, Boston (MA) Traveler, “Carens Says,” pg. 21, col. 1:
He will shoulder a lot of chores, do the “paper work” in the A. D.‘s office, besides helping on the rubber chicken loop.
 
17 June 1941, Portsmouth (NH) Herald, “Sports City: Corporal Tony Proves Ace Recruiting Officer” by Don Stewart, pg. 8, col. 3:
Those who are used to batting in that minor league with a major league indigestion average—the league better known as the “rubber chicken, canned peas, and rubber lung” circuit because of its combined menu of food and fiery oratory—are used to the spiels, harangues, and soap box cynicism from various silver-tongued, fuzzy tongued, and two-tongued speakers.
 
14 November 1941, Boston (MA) Traveler, ‘Trek to Troy for Trio” by Carens, pg. 34, col. 3:
How close we are coming to the “Rubber Chicken Loop” is driven home by the fact that Swede Nelson, Dan Kelly, and myself are performing at Troy, N. Y., tonight.
 
16 December 1941, Gettysburg (PA) Times, “Sports Roundup” by Hugh Fullerton, Jr., pg. 3, col. 3:
Popular story on the rubber chicken circuit tells how Bernie Bierman discovered one night that the hotel where his athletes were staying was on fire.
 
8 January 1947, Greensboro (NC) Record, “Butts And McMillin Compete WIth Favorite Jokes In Banquet League” by Oscar Fraley, pg. B-5, col. 2:
NEW YORK, Jan. 8.—(UP)—The rubber chicken circuit is at its indigestible height today and two of the smartest napkin knucklers on the knife and fork loop are Wally Butts and “Poor Ole Uncle Bo” McMillin.
 
Google Books
Baseball Complete
By Russ Hodges
New York, NY: R. Field
1952
Pg. 181:
Much of Vitamin Veeck’s time was spent on the rubber-chicken circuit, as an after-dinner speaker on the virtues of the Cleveland nine.
     
Google Books
The Dolphin Guide to Los Angeles and Southern California
By Bill Murphy
Garden City, NY: Doubleday
1962
Pg. 127:
This is known as the “rubber chicken” or “turkey” circuit. It can be a horrible experience, as I can attest.