Hearkening to the higher frequencies of certain Language Log posts, Grant Barrett at Double-tongued Dictionary has an entry for dog-whistle politics, including a 1997 sighting from New Zealand:
1997 The Dominion (New Zealand) (Dec. 16) "Election Fight On Race Issue" p. 8: Labor's spokesman on aboriginal affairs has already accused Mr Howard of "dog-whistle politics"—in rejecting a race election, he actually sent a high-pitched signal to those attuned to hear it.
Grant has found a 1988 U.S. example of a somewhat different sense, the "Dog Whistle Effect" in polling:
1988 Richard Morin Washington Post (Oct. 16) "Behind the Numbers: Confessions Of a Pollster" p. C1: About 15 percent more people were "very happy" when the alternative was being merely "fairly happy." Maybe they were really that happy, or maybe the pollsters offered them unacceptable choices. Anyway, researchers call this the "Dog Whistle Effect": Respondents hear something in the question that researchers do not.
And he gives a 1989 example from advertising:
1989 Russell Smith Dallas Morning News (Apr. 9) "Has Mtv Sharpened Its Edge?" p. 1C: The spot, which has no obvious reason to exist, is like a secret signal, a dog whistle blown on the thirtysomething frequency. Come back, MTV beckons, in a language it hasn't spoken in years.
A quick LION search suggests that in the 19th century, dog-whistles (then presumably not the silent type) were sometimes stereotypic exemplars for inconsequential sounds. For example, this from the 1877 edition of Philip James Bailey's 40,000-line epic poem Festus:
Lucifer.
20485 Now therefore would I wager, and I might
20486 The great archangel's trump to a dog-whistle
20487 That whatsoever happens, worse ensues.
Festus.
20488 Even the unwise may prophesy, now and then.
And Sir Henry Taylor's Philip van Artevelde:
Posted by Mark Liberman at January 10, 2008 07:00 AM126 On my soul,
127 If there be justice I can render him
128 He should receive it from my ready hands
129 Although his voice in Council were as small
130 As a dog-whistle.