As I understand it, the main question posed by is "Do we need a "lightweight abstraction layer""?

Equivalently, as I understand it, the question is "Can we attach the authority-related attributes directly to a person / document / place or do we need some separate resource for them?"

My answer is (a) yes we do need a separate resource for them and (b) we can do that very easily.

(a) yes we do need a separate resource for them
For Instances where the carrier is digital and we have a URL the instance, we already need to make a differentiation between our rdfs:Resource for metadata about the instance and the rdfs:Resource that is the instance. Otherwise we can�t differentiate, for example, between the HTML version of the FRBR report and it�s HTML catalog entry. Once we've made the distinction for digital Instances, having a different genre of 'thing' for our other entities in our model is a recipe for confusion.

(b) we can do that very easily
In the foaf vocabulary there is a foaf:Person, a foaf:Document and foaf:primaryTopic which are what they say on the tin and have, I believe, exactly the semantics we�re looking for. foaf:Agent and geo:Point are obvious contenders for Organisation and Place.  We can reuse them directly or via semantic sugar (a la madsrdf:hasCloseExternalAuthority). There are other namespaces, with coverage of these areas; pretty much any of them is better than rolling our own. foaf also has the benefit of already being used by VIAF (and I believe it works for them, I've certainly not seen any complaints). 

[I was going to use some examples, but it�s down as I write this...]