Q:

Can "thereof" take a plural noun as a referent? 

Just as the title states.  I ask because most definitions only state "of that; of the thing just mentioned," but not of "things."  Moreover, is "thereof" a pronominal adverb?  And, as such, does it function partly as a pronoun?  
Thanks for your time!  

A:

Sure.  As you might expect, it appears in legalese as from New Jersey statutes annotated:

The mayor of any such city adopting the provisions of this act shall have the right to veto all the ordinances of such board of public works and board of water commissioners and also to veto such resolutions and actions thereof as he is now empowered to veto by the charter of such city....

Thereof is a compound preposition, meaning above

of those boards just mentioned

where it doesn't function as an adverb but as a modifier of resolutions and actions.  Of course, it's possible for thereof to modify a verb as in the translation of Wittgenstein's famous diktat:

Whereof one cannot speak, thereof one must be silent

that is, "Where you can't make sense, there you should be quiet."