I can sight read simple pieces, but not intermediate or difficult songs proficiently enough to play live. But when I have been caught in that situation, I would simply sight read the bass line (simple since its only one note) and use my ear to anticipate the upcoming/corresponding chords based on what has been played. I'll try to keep an eye out for any weird chords or accidentals that may come up in the right hand, but for the most part thats how i cheat my way through it.
Tj