Don't think the liner Idea on the spring will hold up long, it will flex off as the springs move.
I doubt that, the liner is slightly rubbery. It can't be less flexible than regular paint. Plus the springs are several feet long (uncoiled) and the torsional flex they see per square inch is actually pretty small. I'll ask the coating guys though if they have done springs before and if it will work.
It would just be handy to get the springs squirted at the same time as the bumper. I like the idea of liner on the bumper for durability, but probably what would look best would be just regular paint that matches the old bumper color. The liner can be color matched but even if it's the same color it won't be the same texture, the fenders and bumpers are glossy.
BTW in case you wondered why I'm bothering with coating the brand new springs, it's because the paint they were originally painted with already flaked off in the box before they were even delivered to me. Almost half the spring is just bare metal. I gotta get them coated with something and obviously spray paint didn't stick to them so I'm looking at other options. It's a real pain in the ass because I don't have a yard where I can strip them and stuff, I need someone else to do the work (sandblast or strip and paint or coat etc). That is one nice thing about OEM or Eibach springs, they come ready to use.
I think powdercoat would be really cool on the bumper if they could match the fender flare color, but might be expensive.