Wouldn't blinking with your third eyelids give it away though?

Appropriate username.

On topic, though, I tried it, and changing what I'm focused on doesn't change my pupil size. But maybe I wasn't going far enough? I just looked in a mirror, focused on a spec of dust on the mirror, then changed focus to the image of something behind me, and my pupils didn't change at all.

Instead of focusing like that, think about crossing your eyes but keep them in place. It is like changing the depth of focus, but different.