An alternative here that doesn't rely on setting the file position is to use the ntdll function NtSetInformationFile().

http://undocumented.ntinternals.net/UserMode/Undocumented%20Functions/NT%20Objects/File/NtSetInformationFile.html

It may be a bit hacky, since there is no link library for it as far as I know, you'd have to pull it out of nt.dll manually.