how do you select a center channel to match when the company only makes monitor speakers?

I have a pair of harbeth supper plus speakers ...

Are those for dinner music? :D

Seriously, how about a pair of P3ESRs for the center channel? They'd share your other Harbeth's tweeter and driver types, general dispersion pattern, cabinet design, and crossover philosophy. Those are probably the most important factors in creating a uniform soundfield.