Unfortunately we don't have an exporter yet for Blender 2.8. The author of BRIAGE (
http://jujumatic.free.fr/index.php ) says he's working on it and hopes to finish by the end of the year.
As for back porting your 2.8 model to 2.79, that is harder. Right now I have both 2.8 and 2.79 installed ( separate directories in \Program Files\Blender Foundation ). And I have found the easiest way to move models between them, in both directions, is with copy and paste. In 2.8, select your model and Ctrl C to copy. Then in 2.79, ctrl V to paste it in. Materials and texturing don't always move intact, but its better than starting over.
Wayne