I want to do exactly the opposite of what you do. In other words, if I’m using it as an adjective, I would do what one has always done when two words form one adjective–use a hyphen. It’s an on-line project. I am also out of step with the rest of the world in that I see two words, period, when the expression is used as what you call a predicate complement: “I am on line” is how my brain would write it.