I am assuming the Node-based Platform SDK is just a wrapper (with some validation/massaging) over a REST-based API. Is that the case? If so, will the REST API specification be published at some point? That would enable people to access the Model API from their current (non-node) stacks.
Your assumption is correct and the specification will most probably be published at some point. So far we didn't do that for two reasons.
The api is a lot more generic / low level then the sdk. So a lot of concepts need explanation. (in contrast to the sdk the api doesn't describe concrete concepts like entities, just abstract ones like objects, lists, properties and references)
We will be iterating a lot on this internal api as it is used by several other products as well. So we do offer backward compatibility in terms of the sdk, but we don't want to offer the same level of support and backward compatibility on the api itself yet.