I've never been happy with the RD loops on bikes with the seatstay routing; the cable doesn't bend around far enough to isolate the effect of the RD's movement nearly so much. Kinked ferrules more common.

I'd be inclined to experiment with a V-brake noodle; for whatever reason they seem to have surprisingly low friction for the small bend radius (at least with a fresh liner)... it must be that they're rigid.

If you can get hold of a really long one (some are 4" or more), you could straighten the terminating end and poke it in the cable stop on the frame, running down the seatstay a couple of more inches and then bending back to almost horizontal a couple of inches above the cassette (I'd go behind the seatstay). This'd give you a 180° bend of slightly smaller radius (a C rather than a ?)... might be an improvement.

V-brake noodles can do some cool stuff; they're God's gift to unreversed sidepulls on the rear of stepthroughs.