If HP does add white to the R2R latex, I would assume it would have the same/similar delivery system as the R-Series flatbed. When white is not in use, the white can be set to recirculate to keep the ink from separating, and the white head can be completely removed and is stored in a mechanism that keeps the head in motion. This is different than the FB series in which white would have to be purged from the system, and required a tremendous amount of ink to flush, and replace with LC or LM (can't recall which).
It would seem like they would need to slow the printer down when printing white, especially if white is printed underneath color to make sure that the white cures properly. They may be able to accomodate this by adding another curing zone, but that seems unlikely. I would expect a big price increase to accommodate this addition. If they can get it right, it would be a good feature, but getting the opacity and durability is going to be a tall order for latex R2R in my opinion. From what I've seen on the R-Series, the white looks amazing.