Can I declare that a weapon is wielded in my off hand, even if my main hand is free?

The usecase is that I have a class feature (Swordmage Warding perhaps?) that gives me a shield bonus if I have a free hand, and I'd like to wield a rhythm blade in my off hand to boost that shield bonus.

Can I be considered to be wielding the blade in my off hand even if my "main" hand is free? Or must I be dual wielding for a weapon to be considered to be wielded in the off hand?