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?