Update: In the meantime, there exists an official, fully elaborated proposal for exactly this kind of feature in schema.org:
https://www.w3.org/wiki/WebSchemas/PropertyValuePairs
See the examples in the W3C wiki for details.
The same mechanism will soon be available from the GoodRelations namespace.
Note that it does not yet work in live Web sites - you will have to wait for the proposal to be added to schema.org.
There is not yet an official statement on whether and when the extension proposal will be considered, but there are quite some use-cases for this, which is why I am optimistic about it.
Best wishes
Martin