Providing SQL access to flat file (proprietary) database
Possible solutions:

Use SSIS package registered as a view.
Downside of this option is that any where conditions are not pushed down to SSIS, so dotNet will have to read all the data. Of course you can create multiple views - like last day only, aggregated values, etc.
Walkthrough: Publish an SSIS Package as a SQL View
Use a CLR Table-Valued Functions
This way you can pass parameters to underlying dotNet code
Use PolyBase.
This would require a lot of added infrastructure and will allow you to split processing the file to multiple servers (or in Azure).

On the other hand I don't think that compression you are using will be much better than columnstore compression, so I would advise to reevaluate storing this in SQL Server itself.