The entry for pig in the Oxford English Dictionary has an entry for the phrase, in a pig's eye:
colloq. (chiefly N. Amer. and Austral.). (in a) pig's eye (also ear, arse) : used as a derisive retort expressing emphatic disbelief, rejection, or denial.
The listed uses are:
1847 J. J. Oswandel Notes Mexican War (1885) iii. 163 Mr. Nicholas P. Trist?is on his way to negotiate with the Mexican government to make peace. How are you peace-peace in a pig's eye.
1876 Oakland (Calif.) Daily Evening Tribune 17 Mar. 3/7 'Bought this mare for $16?'. 'In a pig's eye you've bought her for $16'.
1951 E. Lambert Twenty Thousand Thieves 322 'Pig's arse to that!' another voice cried. 'A jack-up-that's the shot.'
1968 W. Garner Deep, Deep Freeze ix. 110 'One stops short of probing the private lives of people for whom one has a regard.' 'In a pig's ear!' she said vulgarly. 'If duty called you'd have a man under the bed on my honeymoon.'
1992 O. S. Card Lost Boys (1993) vi. 154 'She must not have any idea of the effect of her words then'?. 'In a pig's eye.'
So the first recorded use was in 1847, and by this time the OED says that it was already being used as a "derisive retort". As the phrase is chiefly from North America and Australia, it is highly unlikely that this is Cockney rhyming slang. However, the article does say that "in a pig's arse" is an actual variant. One of the included uses (see 1951, E. Lambert) uses arse instead of eye.
http://english.stackexchange.com/questions/39902/origin-of-in-a-pigs-eye
IN A PIG'S EYE - "Never, highly unlikely. Whether the originator of the saying meant that a poor idea was something to put in a pig's eye or that it would look bad to a pig's eye is a matter of speculation. As an expression of scorn the expression was picked up in 1872 by Petroleum V. Nasby (David Locke) in one of his satirical newspaper columns: 'A poetical cotashun.which.wuz, -- 'Kum wun, kim all, this rock shel fly From its firm base - in a pig's eye.'" From "The Dictionary of Cliches" by James Rogers (Ballantine Books, New York, 1985).
http://www.phrases.org.uk/bulletin_board/10/messages/867.html
It's probably not rhyming slang. The phrase exists in French and Spanish (at least) too. 'Dans l'oeil d'un cochon', and en 'el ojo de un cerdo', respectively. It may be that those languages have borrowed it from English, but unlikely. In Spanish there is a variation "En la parte inferior de un cerdo", which you can translate for yourself.
http://english.stackexchange.com/questions/39902/origin-of-in-a-pigs-eye
