Is it possible to get shell properties for an item not in the shell namespace?
(Have some experience in Property Store handlers) How I see a solution:
Get PropertyStore handler CLSID for your file extension. You should use 2 regkeys key:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\PropertySystem\PropertyHandlers\.yourextHKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\PropertySystem\SystemPropertyHandlers
Create two objects with CoCreateInstance
If you have 2 object you can combine them into single object with PSCreateMultiplexPropertyStore
Query for IInitializeWithStream (also you can try to query IPersistStream).
If the PropertyStore object supports IInitializeWithStream/IPersistStream: you are lucky - just init your object and query the properties you need. If does not - you still have (dirty) variant to create temporary file and then use IPersistFile.