How do I use the WIC API to read/write custom EXIF data?

Question

I have gone through the documentation at: http://msdn.microsoft.com/en-us/library/windows/desktop/ee719799(v=vs.85).aspx

As context, I am trying to encode in the JPEG-XR format and I want to emulate GDI+'s SetPropertyItem, GetPropertyItem functionality.

I basically have 3 questions:

1. If I want to add a custom property to the exif header what is the correct query path?
2. Can I use a custom ID, say {ushort=1111} to identify it and how do I verify if an ID is already defined?
3. Is this the same as the id field of GdiPlus::PropertyItem?

For example, is the following valid:

PROPVARIANT    value;
value.vt = VT_LPWSTR;
value.pwszVal= L"Metadata Test";
hr = piFrameQWriter->SetMetadataByName(L"/ifd/exif/{ushort=1111}");

This code block succeeds, but when I try to read back the same metadata using:

IWICMetadataQueryReader *pQueryReader = NULL;
if(SUCCEEDED(hr))
{
    hr = piFrame->GetMetadataQueryReader(&pQueryReader);
}
if (SUCCEEDED(hr))
{
    PROPVARIANT value;
    hr = pQueryReader->GetMetadataByName(L"/ifd/exif/{ushort=1111}", &value);        
}

This returns E_INVALIDARG error.

I'd appreciate some help in understanding how this works. I feel like I have not understood the documentation correctly.

Thank you.