I always have 'participation' as one of my grading points, & am very explicit about the fact that a 'behind-in-a-chair' is not participation. However, it is how you define participation that takes into account your obstacles [above]. I widen participation to include asking questions, the bulk of which qualify (yes, there can be 'dumb' or attention-seeking questions). I have found (over the past four years) that even the most reticent students participate by the end of the quarter/semester.