Well, went back to emissions test facility, crossed my fingers, and passed fine. Happy ending. I would submit readings, but IL now measures in "grams" not gm/mile like the last time I took the test. Numbers are useless to even compare to previous test. In any case, I was pretty comfortably within the limits. So if anyone does a search in the future, a 1984 euro W126 500SEL with aftermarket catalytics will pass IL-DOT's super fancy "real world driving" emissions test.

I appreciate what the IL-DOT is trying to do simulating real driving conditions as opposed to static speed/rpm. However, it is very unfortunate that you don't get an actual % measurement out of it. Kind of a free engine diagnostic that most DIYers can't easily get.