There is a simple hack to this:
Insert a hidden input field in the form with an entity which only occur in the character set the server your posting (or doing a GET) to accepts.
Example: If the form is located on a server serving ISO-8859-1 and the form will post to a server expecting UTF-8 insert something like this in the form:
<input name="iehack" type="hidden" value="☠" />
IE will then “detect” that the form contains a UTF-8 character and use UTF-8 when you POST or GET. Strange, but it does work.