I know that you can turn any DA server into a REST server, but is it possible to use DA to connect to another API like maybe Salesforce. What Tdaschema does for connecting to a database layer are there any classes that could be used to connect to an external REST server from a DA Server and then map the data back to an RO client?
it would be possible, if you wrote wrapper for that API using RO/DA.
If you are using Delphi/C++ Builder you can turn any REST data source into a FireDAC dataset using the Enterprise Connectors from CDATA. https://www.embarcadero.com/products/enterprise-connectors
I think there may be a way to purchase the ones you need separately from CDATA.