I don't know if this really counts as a "plug", but there was a time (I want to say 1974 or so) when some of the ABC shows would mention what show was on next (e.g. on Password, Allen Ludden would tell the viewers to stay tuned for Split Second). I don't think they all did it, especially as, IIRC, LMAD was on at wildly different times in different parts of the country - in fact, I remember Monty Hall plugging a time slot change for the show, "but we'll still be on at 1:30 on the west coast."
I also remember Jim McKrell mentioning on an episode of Celebrity Sweepstakes that Wheel of Fortune was premiering that day, even though it had already been on the air for at least a week.