Typesetting oxidation state and charge simultaneously
My personal preference in this situation would be to put square brackets around the whole ion and put the charge outside. 

$$\ce{[Cr^{VI}2O^{-II}7]^{2-}}$$ gives:
$$\ce{[Cr^{VI}2O^{-II}7]^{2-}}$$

This is sometimes done with polyatomic species anyway (particularly metal-ligand complexes) to emphasise that the charge is on the whole ion, not on any one particular atom.