Is it possible to have a ferrocenol? I'm specifically talking about having a hydroxy- group on the ferrocene ring. 

Is it simply impossible because the ring is anionic?
I haven't seen any hydroxyferrocene molecules available online. (unless they are a methanol group)
Yes we can have hydroxyferrocene.  See here for a brief summary description of possible synthesis and a nucleophile substitution reaction forming alkoxyferrocenes.