I have confirmed that behaviour, using flash xmlsockets will break deflate+utf8+base64 for some reason that I could not find out. It is a problem with libraries, not with actual fop2 code. So, in the meantime I tried removing deflate from the equation and have straight utf8+base64 coding on the server side, and it worked fine, at least when using flash xmlsockets (I have not tried with https, but I asume it will be similar).
So, I think I will leave that change for the upcoming 2.29, so anyone with wide characters on buttons and using xmlsockets instead of standard websockets will not have issues.