Interesting idea, but it would be harder for me to remember 30 characters of emoji on a keyboard I don't routinely use than 30 characters of text and symbols which have, to me, some internal order, on keyboards I have used for the past...18 years?
I don't know how you get to the emoji keyboard on iOS. It would also be a little annoying having to do this on multiple platforms (I have considered using 1p on Windows at some point; no real argument that a well configured win 7 or 8 is worse than OSX, now).
An emoji password like happyface heart pizza devil hospital trafficlight cow moon is probably going to be more secure than any word/number combination that could be dictionary attacked.
This obviously assumes the site properly encodes Unicode, but also somewhat limits use for most people to devices with the keyboard.