I do not think that it's illegal to charge more to prospective tenants that use an agent's services but would they be willing to pay more in monthly rent than another possible tenant who is not represented? Would the landlord disclose the difference in monthly rent to the prospective tenant? Maybe the represented tenant may just wish to pay the commission to his agent and pay a lesser amount for rent.
The landlord may end up over pricing the rental unit if he adds on the additional monthly fee which may increase the market time on getting the unit rented particularly if there are others in the neighborhood available. It may be best just to say that a tenant represented by an agent must pay the fee for his agent's services.
Just food for thought.
Prudential Connecticut Realty