What about those old-school Schwinn eraser color pads, aren't they non marking? I have never found a mark on any Schwinn rim that had those pads. or you could always go with a disk brake conversion.
No matter what you use, you're going to wear the coating off of rims if you use the brakes. It's not the pads that do this but the abrasive dust and road grime that gets under them that will wear off the finish.
I've replaced rims on bikes that get used hard because the pads wore through the rim itself.
Maybe consider a drum front hub? You could leave the cantis on the fork if you ever choose to go with a non-painted front rim or move the powder coated one to a new project.