Hello,
In my opinion the protection/detection of a file using it's metadata is only a good option/solution if you can guarantee that the metadata cannot be forged or deleted. Even if you use a third party tool to inject metadata to protect documents, this piece of information should be protected against edition (e.g. required password to change advanced properties of a PDF document).
You should also bear in mind that for some protocols the detection by metadata might not be enough for DLP to trigger the detection / response rules. For instance a document might be printed (even if the response rule is configured to block the action) if the only detection is inserted in the File Properties.
Just my 2 cents..