Internally it looks like you are using RadSpreadStreamProcessing for grid.ExportToExcelAsync() if you gave us an optional lamdba to manipulate IRowExporter while you are processing, it would make things a lot easier.
There were a couple other feature requests out there that you closed offering alternatives ways of doing this and we are actually just importing the stream back to RadSpreadProcessing object and then manipulating that way. Adding the Lambda would be a much more efficient way of handling this use case and probably very simple for you to implement.