I've put a lot of pressure on the cables, so far, without any problem, at least not yet.  Have only done some 2 and 3 day trips so far.  we put the harnesses on first.  The foam inserts help keep it off the cables. After the harness is tight, then we attach the dry bag or tent and the handlebar bag.  It is awkward, but without the weight and bulk of the other items, not impossible to install.

My boyfriend uses it on a lefty without any problem.  He loosely attaches the strap to the metal connecting the fork to the head tube.

WaMBR -- what a great idea!  I just found this as I was trying to plan a trip to see Portland OR, ride a few hundred miles, and also visit my mom in Yelm WA.  I'm thinking of flying into Portland then riding this route as far as Ashford, before cutting east to see mom.