There is a special automotive finish available which I'm not calling
paint because I'm not sure what it is.  But auto painting
professionals will know about this. coating.  It seems to be slightly
rubbery and seals wonderfully while protecting front lower valance
panels on vehicles intended for use where road stones and brush are
likely to impact this area.  The applications I've seen had an obvious
"orange peal" texture appearing surface rather than looking like
smooth glossy paint.  That characteristic doesn't sound wonderful, but
the virtually perfect water seal it gives and it's flexible durability
are attractive characteristics which could give just the protection
you'd like.

How to apply it and and what aesthetic plan you'd find pleasing are
open questions.  But for protection, I think that would be excellent
since it's flexible so it won't cause a crack line when the body
flexes a few thousandths of an inch when you hit jarring road hazards
in the coldest weather.  I once filled welded body seams on a  pickup
truck cab where the factory elected to leave an externally visible
seam.  The next winter, those seams on each side cracked through to
the surface.  Amazing.  I suspect that compared to the Vanagon seam,
those welded pickup cab seams were much less mobil, so any material
you use to bridge this seam MUST remain flexible if it is to endure.
Anyway, that's my opinion.

How about a decorative stripe about 3/4 inch wide, centered on the
vulnerable seam?

All the best to you,

John