You can. And I do. The other players do not have to abide. It is a PvP zone after all.
On a side note. I only play on the Excelsior Server. At times the most populated server in the Homecoming group. I go to PvP zones on a regular basis for Temps and Bonuses. In the last year, only once have I had a problem with someone not respecting my wish to be left alone. My point is that, overwhelmingly the people playing this game are decent people who will respect your wish.
Usually you will get killed once before you can get your text out. That's just par for the course. You are playing in a Zone designated PvP.
I see no need for a "opt-in PvP zone." Nor do I see one being made. PvP in general is incredibly under used in CoH. There are too few PvP players playing CoH.