No it would not be OK -- chiefly because it would paint you as being bumptious to the point of rudeness. Would it stick in your throat to call a Frenchman "Monsieur" or a German "Herr"? Would it do violence to your conscience to call the Queen of England "Your Majesty"? Are you too pure to address a letter to your congressman as "The Honorable Jackson Blowhard"?
If your detestation for the office of Catholic pontiff is such that you can't bring yourself to observe the small courtesies of polite society (you have no problem with expecting me to address your own clergy as "Reverend", I'm guessing), then please stay home and save yourself from the assaults of this sinful world.
Acually, you would be corrected if you did. Where I worship there is no distinction between clergy and laity.
The difference would be with my conscience regarding what I see as a blasphemous title versus a secular title.