Jul 26 2016 04:44 PM
That gets tricky as the Excel Services Service Application is actually performing the refresh in this case. It would typically use the Secure Store Target Application profile configured for data access as designated in the Excel spreadsheet. This is basically how reporting works in Project Server.
I haven't tried to do this setup with Office 365 but I think you can set up a SSS profile in the SharePoint Admin portal. I'd be curious to see if it works with the gateway. If it does, this opens up some new possibilities.