Billy Mernitt, you have just knocked my favorite crutch away. My best excuse for procrastination. The reason I end up spinning in circles and efficiently avoiding forward progress on my screenplays (or any writing, really.) I don't have to get it "perfect."
According to Mr. Mernitt, who has every reason to know, it's not only OK to break the "rules" of screenwriting, sometimes it helps you achieve the one goal that is paramount for any screenwriter: make them empathize with the protagonist. Get them to feel. If it takes parentheticals, unfilmable asides about the character's internal thoughts and intentions, or lots of ellipses, then so be it.
Thinking about it, I am reminded of something I learned when studying poetry in college. It struck me while reading Sonnet: On the Sonnet by Keats. The Sonnet is a very structured form. Not the most difficult, but still fairly rigorous. (I laugh when people complain about screenplay form being too limiting.) In this sonnet, Keats breaks the rhythm in one line - and does so with the explicit intention of making the reader pause, of drawing attention to the one line.
I learned two things from that poem: 1) Let structure serve a meaningful function. The actual rhythm contributes to and/or echoes the content. 2) Drama and meaning can be derived from breaking the form - but there has to be a form there to break for that to work.
So I think my feeling about using such things as internal unfilmables, is that they should be used consciously. They should be placed in essential spots, to break the form a little, change the pace, make the reader stop and think. But to achieve that effect, the rest of the writing has to be fairly clean and straightforward.
I know the examples on Mr. Mernitt's blog include a script that seems to go beyond the selective use idea... but I also found it a difficult passage to read. Maybe it reads differently in context.
Have I come full circle? Have I just proposed something even more difficult than perfection - perfection with intentional flaws? Shhh. I don't care. I need to go write.