Prevent wrapping custom element in p element

I am making a plugin which produces a custom element. I want it to behave like the hr element: If the cursor is inside a non-empty p element, it splits the p so that the hr is not inside a p. If the cursor is in an empty p, it removes the p. How do I get it to do this?

I can configure it to make my element empty like this:

CKEDITOR.dtd.$empty.myCustomElement = 1;

I ought to be able to make it behave as I describe above like this, but it gives an error message instead:

CKEDITOR.dtd.$block.myCustomElement = 1;

Any suggestions? Thanks.