I think you're correct about it being UTF-8.
Is it possible to change the unicode/Wide versions of GFLSDK so that it works correctly with the UTF OpenEXR DLL (and any other plugin DLL's, if possible) - converting between unicode and UTF-8 where necessary. Otherwise it means you have to use the ANSI version of GFL if you want to support OpenEXR or any of the other plugins.
In case you need a routine to convert between Unicode and UTF-8, check out http://weblogs.asp.net/kennykerr/archiv ... utf-8.aspx
Let me know if you want me to test anything.