This is a basic functionality for the Image::ExifTool Perl library, but unfortunately the .NET wrapper goes through the exiftool application. Here, the only thing you could possibly do would be to create some sort of pipe from your bytes array (which may be possible with .NET, I don't know), but then you would probably run into troubles with the .NET wrapper not supporting pipes (I'm guessing the wrapper probably uses pipes for the command-line arguments, and if so you probably can't pipe the images too).
So without knowing how the .NET wrapper works, I am guessing that it won't do what you want.
You could, however, launch ExifTool directly piping the image if you can figure out how to do that, without the ExifTool .NET wrapper.
- Phil