I would hesitate to do this on 1x19 shrouds, just because of the point bending in an inflexible cable. Then again, lots of people hang a hammock off of their forestay.
The benefit of "rat-steps" is that they are rigid, which transfers the force directly to the stay. Rope ratlines pull sideways on the stay. This magnifies the tension force necessary within the stay to keep you from falling. I don't imagine this would be a problem on a boat with 1/4" shrouds or larger. Anything smaller than that and I would be scared you may stress the cable unduly.
Also, if you insist on the rope ratlines, it would minimize the load on the cable if you kept them loose. Remember, line only works in tension while wood works in compression
Here is an illustrative example.
Let's say your mass is m=100kg.
Assume gravity is g=10kg*m/s^2
Lets assume you are in a hammock. If the lines from the hammock meet the tree at a 45 degree angle:
The tension on the line is 1414 N.
Okay what if the hammock is really tight and the lines meet the tree at 20 degrees?
1000/sin(20)= 2923 N
Now what if the hammock is super tight and the lines meet at ten degrees?
1000/sin(10)= 5759 N
If we extend this thinking a little, we can apply it to your ratline question.
Instead of a hammock, lets say you have a ratline with your weight on it.
Instead of trees, lets say that you have shrouds.
Now it should be clear why loose ratlines will put less load on your shrouds.
I would use a double constrictor knot
if I were doing it on your boat. I wouldn't do it on my boat though!
A pinrail, maybe.