The knots will still rub against the rock and with you weighting it
will expose the core ruining the rope.
but you are thinking which is Ok. Hooking sideways along a wall and placing a real piece when a crack presents itself then back cleaning it. Never getting higher than an easy step to the ground is a safe way to teach your self and your feet the fun pain they are in store for.
If its just to gain muscle memory for messing with aiders a tree or a chin-up bar is good.
Stemill, where do you live? There are man made aid venues all over the place.
Freeway bridges and retaining walls in the river channels provide cracks.
When I was a young teen we would aid all over the ceiling of my cousins unfinished attic. 2 runners for tying off 2x's, harness, carabiners, daisys and aiders. BITD we did it with swamis and no harness.
I had a block retaining wall in my neighborhood that had short thin parallel cracks to traverse. With a little looking there is stuff all over the city.
Your idea of the rope ladder setup doesn't sound that bad either for basic etrier practice.