IIS7 native module - setting request entity with file handle - c++

Question

So I'm working on an IIS7 native module, and part of what it will do is process some fairly large uploaded files. I'm trying to work on ways to reduce the memory footprint of the module while doing this.
One thing I was able to do with the processed response data, which is nice, is pass open file handles to the underlying system instead of memory buffers by using the HttpDataChunkFromFileHandle chunk types. I'm trying to do the same thing with the request data, but so far no joy.
What I am doing is first I am reading all of the request data, processing it, and then setting the entity chunks in the raw HTTP_REQUEST like this:
HTTP_REQUEST* rawRequest = _context->GetRequest()->GetRawHttpRequest();
rawRequest->EntityChunkCount = 1;
rawRequest->pEntityChunks = new HTTP_DATA_CHUNK[1];
rawRequest->pEntityChunks[0].DataChunkType = HttpDataChunkFromFileHandle;
rawRequest->pEntityChunks[0].FromFileHandle.FileHandle = _requestFile.handle();
rawRequest->pEntityChunks[0].FromFileHandle.ByteRange.StartingOffset.QuadPart = 0;
rawRequest->pEntityChunks[0].FromFileHandle.ByteRange.Length.QuadPart = _requestFile.size();
and returning RQ_NOTIFICATION_CONTINUE.
This results in a 403 response from the server.
If I use a memory chunk instead, it works correctly:
char* bufferOut = static_cast<char*>(_context->AllocateRequestMemory( _requestFile.size() ));
std::memcpy( bufferOut, _requestFile.map( 0, _requestFile.size() ), _requestFile.size() );
HTTP_REQUEST* rawRequest = _context->GetRequest()->GetRawHttpRequest();
rawRequest->EntityChunkCount = 1;
rawRequest->pEntityChunks = new HTTP_DATA_CHUNK[1];
rawRequest->pEntityChunks[0].DataChunkType = HttpDataChunkFromMemory;
rawRequest->pEntityChunks[0].FromMemory.pBuffer = (PVOID)bufferOut;
rawRequest->pEntityChunks[0].FromMemory.BufferLength = _requestFile.size()
So... is HttpDataChunkFromFileHandle just not supported for request entities? Or is there something else I need to do for it to work?
Do I need to set any specific security permissions on the file?