The problem with morphs or other thin walls is the double editing to match the geometry, doors and windows of the original wall.
If it is always horizontal then you could also use a complex profile wall with modifiers so you can stretch the height of the datum (paint).
You could even make with a thin surface in front of the main wall in the composite that you can stretch back down to nothing if you don't want the datum. (i.e. this is done when you want tiles on a wall - but in your case it will be the paint).
This doesn't help with the diagonal - I would use the align texture method Ling mentioned.
Barry.
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