I don't worry about scratches that don't go through to fabric.  To me they're just part of using the canoe.  I've been in too many situations where not scratching the canoe puts me at more risk or is just a huge hassle than it's worth.

Why the aversion to felt plates?  The bike tires is a neat idea with purpose.  I've heard of some folks laying a strip of CF tape along the keel.