I am currently using Ultimate Chrome Flag Extension https://chrome.google.com/extensions/detail/dbpojpfdiliekbbiplijcphappgcgjfn to get extra information about a site (including PageRank).

It is true that it doesn't show this information over the flag icon, but maybe this will happen in the future. It might be a good alternative for people willing to find more about a site than just PageRank (such as Alexa Rank, WOT, location of the IP address)

On a side note: it is definitely not good with getting the IPs of websites that use GeoDNS since it uses a web resolver located in US.