It should be possible to create a subscription to notify the user when any WTPart, WTDocument or CAD Document is shared to a specific Project Container. It should work regardless of which folder the object is shared to. It could even be a folder which did not exist when the subscription was created.
According to CS148719: "The ability to create a notification that is configured on a project such that the notification is issued whenever the project is the target of some share is not available out of the box."
"Reported to R&D as SPR 2200364"
The SPR 2200364 has been closed by PTC with the status: "No Plans to Fix".