Mark Liberman has a most interesting Language Log post about two forms of encoded language, Vietnamese nói lái and French contrepets. The latter is a form of potential punning that depends on imagined malapropism; as Mark puts it:
These are exemplified by phrases like “que votre Verbe soit en joie”, which literally means “may your Word be in joy”, but which expresses a less spiritual message if the indicated sounds (not letters!) are swapped: “que votre verge soit en bois” = “may your staff be of wood”.
The Vietnamese form is (to me, anyway) more interesting:
You can think of nói lái as subversive communication by means of implied speech errors. For example, in the period after the fall of Saigon to the communists in 1975, residents would say of the obligatory picture of Ho Chi Minh that they would like to “lộng kiếng” = “frame (it in) glass”, by which they meant that they would like to “liệng cống” = “throw (it in the) sewer”…
As John Balaban says of Ho Xuan Huong,
the greater part of her poems–each a marvel in the sonnet-like lu-shih style–are double entendres: each has hidden within it another poem with sexual meaning. In these poems we may be presented with a view of three cliffs, or a limestone grotto, or scenes of weaving or swinging, or objects such as a fan, some fruit, or even a river snail–but concealed within almost all of her perfect lu-shih is a sexual design that reveals itself by pun and imagistic double-take.
See Mark’s post for examples. (And here‘s an idiosyncratic but detailed look at how lu-shih works in its original Chinese context.)