Is it possible to add custom attributes to instances of an object, rather than only the reference?

This would be useful for several apps, where you want to create a unique attribute for similar instances, in order to separate them later on in for example, the report template administrator. 

To give an example, this would allow: identification of a certain instance in web environment, positionnumbers as attributes in 3D, designation of an instance (for example: 3 identical manufacturing cells, each for a different type of product)

I believe this is already possible in the on-premise solution, so hopefully this isn't too much of an enhancement to create.