It seems like this issue boils down to the security layer picking up the headers, parameters etc (which will default to default-encoding).
Have u tried the encoding attribute directly on the form authenticator via context.xml placed in WEB-INF?
NO ! Because that problem related ONLY to form authentication.
I read carefully description of FormAuthenticator problem.
But in my issue:
1) no problems with authentication
2) not used form authentication
My problem in general data posting, not in authenticator form.