is it the case that you cannot conventionally nitrate ferrocene because the Fe is too good of a nucleophile compared to the ring?

Although the iron atom is a good nucleophile for some electrophilic aromatic substituions it is unlikely to be the case in nitration.

 

For instance Friedel Krafts acylation will almost certainly occur by formation of a Fe-C(O)R moiety and the acyl group will attack the ring in an endo fashion.

 

Any likely nitrating conditions, even as mild as using for example [NO2][bF4] will almost certainly oxidise the ferrocene to the ferrocinium ion [Fe(eta-C5H5)2]+. This electron poor nature of this cationic species will then make electrophilic attack extremely unlikely