I've got a thread belowstairs trying to document the programs that aired before and after Jack's show on radio ("Bracket Programs?"), but found an interesting tidbit that relates to that.
Selling Durkee-Mower's Marshmallow Fluff (yum!), "The Flufferettes" was a weekly regional radio show that aired in New England starting in 1930 and continuing through the mid-forties, apparently airing "right before Jack Benny". So, that would suggest that early on, probably through a good chunk of the 1930s, local stations would have different programming before the 7 O'Clock slot, at least until the Great Gildersleeve became Jack's lead-in.
http://www.marshmallowfluff.com/pages/history1.html