I race at the velodrome. I rode clipless pedals there for a few years, but never felt entirely confident. I've seen people get bumped (read: slammed), and get one foot unclipped. With no freewheeling and no brakes, it takes intestinal fortitude and a bit of luck in order to stay upright. So I always thought that toestraps still had value for track riding. Additionally, I have a hard time keeping my feet from wiggling when I do a standing start in a time trial and I sometimes pop out of clipless pedals.
So I rode with toestraps on the track for the safety, but always wished that I could ride clipless pedals for the convenience (and the need for fewer pairs of shoes).
This last spring, I had a brainwave. I ride Speedplay Zero Track pedals on the road. (Even though I preferred the high retention force of my old Looks, my cleats always squeaked and drove me nuts, so I switched. The Zero's retention force barely cuts the mustard, but they're quiet and they work.) I have a pile of old junk in the bike room and cobbled together a setup that allows me to have toestraps with my Speedplays when I want them.
So now this setup allows me to use toestraps with my road pedal/shoe setup, and I can take them off when I don't want to futz with toestraps. It's nice to be able to just hop on the bike, do a warmup, and then later attach the straps when I think I need them. I just clip the Strap-Ons to one side of the pedals, clip my shoes in the the other side, tighten down the straps, and go. I even use them on the road sometimes. It's nice to have the option when doing certain workouts.
It's important to note that there is no modification to the pedal or shoe, so I can use this on any bike that has a Zero pedal. My wife and I once did a workout together where we traded back and forth, with one of us resting while the other did a standing start using this setup.
First, the cleat of the Strap-On does not interfere with the cleat on my shoe. There appears to be about two millimeters of clearance between the cleats sandwiching the pedal, and this allows me to unclick my shoes like normal.
Theoretically, this can work with any pedal system that's two-sided. Shimano's SPD is an example. However, the design of the Speedplay cleats seems to lend itself nicely, not requiring significant modifications to any of the parts used to make the Strap-on.
Yes, there's a slight decrease in cornering clearance. On the track, I've brushed the Strap-On in the banking when doing the cat-and-mouse routine in a sprint, but it wasn't enough to cause a problem. (Colorado Springs has 33 degree banking.) But at any speed above a walking pace and on flatter tracks, I haven't noticed this problem. Of course, you can coast through turns on the road. If these weren't simply recycled parts, it would be easy to overcome this problem by designing the Strap-On with a lower profile.
Here's a couple of more pictures.