An HP Velotechnik Streetmachine GTE with no chaintubes.
As far as I can tell, no one else has done this.
Or they were just too lazy to write about it like I was for a few months.
Chain tubes aren't so bad really. They are just another part there wears out frequently. When they are new, they add very little friction. Unfortunately, they need to be replaced at least once a year depending on how much you ride. You also have to break the chain to replace them. There is also a little extra noise that bothers some people.
I wanted to use a proper over/under idler to make sure the chain goes where I want it to. This bike does not have any way to mount one around the steering tube. There is one M6 braze on there for a bracket to hold the return chain tube in place.
I tried the multipurpose upgrade kit from TerraCycle. Their special bracket only allows you to mount the idlers above or below the tube. It was barely usable to mount the idlers above the tube in front of the headset. I would just occasionally hit them with my leg. I just had to be careful. Below the tube made the chain interfere with the wheel. Behind the headset under the tube interfered with how I frequently have to park.
I figured out through discussion with TerraCycle that I should be fine using a derailleur pulley as a return idler mounted on the braze on for the return side chain tube bracket. At rest, the drive side of the chain touches the idler. Under load, the chain passes perfectly above the idler and below my leg and the handlebars.
I've been riding it like this for a month and a half. I'm satisfied. It probably helps that I have an internally geared hub so the chain does not have to move around when I shift.