Cover up hairline cracks?

You might be able to do it with some extra thin cement like Tamiya's.  It should flow into the crack and allow you to push the seams together and hold them while it cures.  

No mater the method you use to fill or close the crack, I don't see a way the seam will be invisible with repainting the seam. Welding the seam usually causes a small amount of dissolved plastic to squeez out that has to be scraped or sanded to make the seam flush.

The model being white will absolutely NOT help, just the opposite.

I don't see how you could ever press the sides of the crack together and not be able to still see it.

Over time dust, dirt, grease will get into that crack and it will begin to show even more.

Touch N Flow applicator and Plast i Weld will do the trick but, more than likely the repair will be visible requiring sanding and re paint.

Maybe some Future with white acrylic paint mixed in?