We don't include it in the "give me all the keys" and we make it binary in PSD to give a bit of obfuscation for the data.Use the following routine instead. I believe the type is a Handle in C land. The "desiredKey" would be the runtime id of XMPMetadataAsUTF8. The desiredClass would be classLayer.
SPErr PIUGetInfo(DescriptorClassID desiredClass,
DescriptorKeyID desiredKey,
void * returnData,
void * returnExtraData)