If you're not setting the value of a hidden field from an ActionForm property, you may as well just use the <input> tag instead of the <html:hidden> tag.
The following should work:
<input type="hidden" value="<bean:write name='messageText' property='messageText'/>" >
Since
Struts uses the name property of an <input> tag to match to the same property on the ActionForm, it will get set properly when the form is submitted.