Yes, you can use metadata files to define the resources that you want to expose through your published OData service. In fact, metadata is a crucial component of OData, as it provides a standardized way of describing the structure and semantics of the data that is being exposed.
Mendix does not allow creating fields with the "Edm.Guid" type through Studio Pro, but you can define the field in the metadata file that you use to generate your published OData service.
The Metadata is available with a published odata service right?
Check the export fieldset the second item for ‘metadata’.
In this metadata, all the entities and fields (attributes) you have exposed are available.