Your first and second pictures are confusing my brain.  At first glance, with the workbench in the background, it looks like these things are giant until you realize that they are sitting on top of another bench.  

I used draw bores on the stretchers on my workbench legs.   I followed Chris Swartz's directions when I built my bench.   Highly recommended and pretty easy too -- with no glue required.  You basically drill the hole in the leg (or whatever) from one side and into the other side of mortise, slide the tenon in and use the drill bit center or lead screw to mark the center on the tenon.   Remove the tenon and drill a hole into the tenon ~1/16 (+/-) closer to the tenon's shoulder and when you drive it home, it pulls the joint tight.  You may have to experiment with how far to offset the hole in the tenon, depending upon the size or scale of the tenon and the size and stiffness of the dowel.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.