This seems to me to be a kind of rhetorical figure, but I cannot find a classical term for it in Silva Rhetoricae. Examples include the following from Tristram Shandy (Vol. 2 Chap. 24):
I define a nose, as follows,—intreating only beforehand, and beseeching my readers, both male and female, of what age, complexion, and condition soever, for the love of God and their own souls, to guard against the temptations and suggestions of the devil, and suffer him by no art or wile to put any other ideas into their minds, than what I put into my definition.—For by the word Nose, throughout all this long chapter of noses, and in every other part of my work, where the word Nose occurs,—I declare, by that word I mean a Nose, and nothing more, or less.
This of course makes it quite impossible not to read nose in the book as also referring to penis.
Another example is an advertisement for Joyce’s Ulysses in America, citing the 1933 decision of the United States District Court for the Southern District of New York that the book was not obscene—which advertisement boils down to “Hey, Buddy, wanna buy a dirty book?”
Yet another is the line “this is not an offer of prostitution” on Web sites, which is an almost infallible indicator that the exact opposite is the case. (For a g-rated account of the use of this line, see Knoxville News-Sentinel story about Craigslist.)
And finally, though it is perhaps not quite exactly the same thing, I would point to the classic device of humorous fiction—dating back all the way back to Cervantes and also seen in Mark Twain and Jerome K. Jerome—of insisting that it is not fiction but rather literal truth, which insistence goes on safely within the fictional compact by which readers agree to be entertainingly lied to.
What can we call this figure? Simply terming such usage facetious or disingenuous seems to fall short of designating such flat opposition between the literal and the intentionally conveyed meaning.