Yes I'm talking from a technical standpoint. They both have a very high and low range although granted Chester's high range is a lot more raspy than Plant's is but I could imagine the two sounding very alike if Chester attempted to do so. Character wise they are completely different, but vocally and technically they are around the same.
Apparently Robert Plant caved, but this is still from The Sun. - http://www.thesun.co.uk/sol/homepage/showbiz/music/article1735624.ece