I just came across this challenge, and in spite of it being from a long time ago, I really enjoyed puzzling it out. I suggest the following to tie in to the harness in the middle of the rope:
- Tie the alpine butterfly loop
- Flatten out the loop to form two bights, one at each end, as shown below
- Pass one of the bights through the loop on the harness
- Use the two bights to tie a zeppelin bend
In the first picture, the line at the top of the picture going out over my legs from the alpine butterfly represents the climbing rope, the green cord represents the harness loop, and you have the two bights with one passed through the harness loop. The end result is shown in the second picture. You end up with three little loops around the knot (two from tying with two bights and a third from the line between the two), but it should be good and strong, and it's quick and easy to tie and untie.
