I have been thinking of trying this for a while, but never have. Through loose tenons. I would keep them square instead of rounded. Seems like it should work. The only problem I can think of is that you would need to make sure that the tenon stock to matches in color to the adjoining rail (if a lot of the tenon is exposed).
I expect to get a few "just use integral tenons for this", but obviously I've considered that already and choose to explore "loose tenons".
Ever tried it?