Observation #1: we’re probably spending way too much time on this 
. But I enjoy a good design challenge, so……
Observation #2: your current brackets should work just fine if you compensate for the reduced radius as you’re mounting the tube in the mount. Mark the entire length of tubing where each bracket will be when straight. So essentially equidistant marks at length ‘x’. And when you mount the tube in the brackets around the loop, make sure the marks line up with the bracket. That will result in a slight bump in the tube when curved, which will flatten out when the drag chain becomes straight. No need for the tube to move freely in the bracket in this case.
