Most the time if there is data published for any SWQL Studio items or even how to query data in the Orion tables it is done on the Orion SDK wiki Home · solarwinds/OrionSDK Wiki · GitHub
Most of what is found there will tell you how to get to data that is in Orion with the SDK and SolarWinds Information Service (SWIS). DPA and Orion talk via jSWIS or SWIS for java.
I find some internal documentation on jSWIS items for DPA only. As far as I know, there isn't a way to query these items from SWQL studio. The tables in the Orion Database that start with DPA_
are mostly very basic tables that are used for the connection and relationships of DPA instances to applications in SAM or Orion objects such as nodes.
The data you are looking for when displayed in Orion is polled with jSWIS from the DPA database repository. So the only way I know of that you can query this at the moment would be to write SQL query items against the DPA repository database. I am not sure what platform your DPA repository is (SQL server, Oracle, or MySQL) but, here is a report that sounds close to what you are looking for that is for a DPA SQL server repository database.
You are correct in what you say on the API for DPA. So far it was mostly centered around DPA Management. Perhaps you can log a feature request for data via the API or adding jSWIS items for DPA data to the SDK wiki I listed above I could see both being good additions.