RDF mapping of a single node?
Some consider it a best practice to create RDF mappings per a given content type but to also include a unique vocabulary for the Homepage node.

I suggest creating it for the Homepage by giving it a unique content type (you could clone an existing one and change by need).

This solution saves overriding the already RDF-mapped previous content type of the homepage node; An overriding that can be tiresome to create and maintain.