I really like the idea of actually using some of those wacky unicode characters for something.
Unfortunately how to input them is highly platform dependent. Looking at the wikipedia entry the important thing to take away is the code point: U+2E2E then usually there is some way to input code points numerically, here’s a guide for windows.
If you are on a linux system I’ve had great fun with this xcompose setup, it also requires switching input method to xim which is probably pretty distribution dependent. But after doing the setup inputting ⸮ is just: compose key + question mark + less than sign. And you easily get access to all sorts of nice symbols: «, ≤, ∑, ∫, ¿, ‽, 
, ẻ, 
, 
, 𝖌, 𝕲(who doesn’t like fraktur?). This actually turned out to be pretty useful when I had to explain math over email for an introductory class where the students didn’t know LaTeX.
As for whether a sarcasm mark is useful, the fact that we have developed visual and auditory queues for sarcasm in speech suggests that we find it useful and necessary. And I confess to not expending the same effort when reading an internet comment, as when I read a piece of litterature or even an essay. Furthermore I believe general usage would rather be occasional underscoring of sarcasm, as opposed to a consistent markup of every single occasion where sarcasm or irony is used. Though perhaps it should be required in an attempt at defeating Poe’s law.
