Following the idea of a one-sided die and keeping close to the wording of alea iacta est will probably be difficult. Contrary to popular opinion, alea does not mean "die," but rather "game of dice" or "game of chance." Roman dice games were usually played with several dice, which were either six-sided tesserae (very similar to modern six-sided dice) or four-sided tali. It is all laid out in Lewis & Short's amazingly encyclopedic entry for alea.
(This is also why "the dice have been cast" is a perfectly appropriate translation for Caesar's famous locution, and "the die has been cast" should probably be considered a hypercorrection and an inferior translation.)
But anyway, I think one option would be to say:
Alea praedestinata iacta est.
... a pre-determined game. (Although that's not particularly sarcastic, I guess.)
Or you could go with the one-sided idea and use a word for die:
Tessera unum latus habens iacta est.
All in all, in a sarcastic tone you could for example say something like:
Alea sane iacta est -- tesseram quidem iecisti unum latus habentem!
(A one-sided die is not a geometrical impossibility, by the way; you can buy them, they are shaped like a Möbius strip. Throwing one as a game of chance is pointless, though.)