I was looking for the relationship between sushi and astral sign characters when I realised reading your link that, while I knew unicode was split in planes, I didn't know about most of them being astral, which deeply satisfied the roleplaying nerd in me.
Pedometer++[1] has donations too — "Generous Tip of $1.29", "Massive Tip of $2.49", and "Amazing Tip of $6.49" (AU pricing).
According to the guidelines[2], "Apps that include the ability to make donations to recognized charitable organizations must be free", and "The collection of charitable donations must be done via a web site in Safari or an SMS". Nothing about donations not to a recognised organisation, so I'm guessing the rule is to avoid sticky laws regarding Apple's 30% non-donation slice.
I imagine cutting off "weird unicode" won't be too hard. For English, it's super-easy - just leave ASCII with letters, numbers, punctuation, etc., cut the rest. For languages with more complex characters it will require some whitelisting I guess, and smart code to disallow stacking diacritics, etc. Sounds quite possible, I wouldn't be surprised if such libraries already existed.
http://mathiasbynens.be/notes/javascript-unicode