Hi Vesa et al.,
I think the pre-course activities are very helpful for this. Once the course officially started, many participants think as exactly you mentioned. They may feel shy to express their views in a forum because of different reasons including language, experience, etc. I can share two experiences that I hope will help you.
Once I attend an online facilitated course and I did not know any participants. During the pre-course session, the facilitator started a Forum discussion, which had nothing to do with subject of the course. It was about the meaning of our names in our languages. Just one sentence or a few only. Almost everyone felt okay and did not have the concerns you mentioned, including me :-) I believe it also helped some participants to get familiar with the Forum and experiment how to post, as it was first time for them. When the course started next week, many participants were participating and I think that pre-course forum helped a lot.
I can share another experience here that we try in the Train-the-Trainer Online Courses. We decided to have the Forum posted to be rated. It seems this is on the contrary with what you expressed as "They may feel their posts are assessed, rated and monitored." Maybe some participants here will remember that we have rating but ONLY POSITIVE "Excellent contribution" and "Like". Of course this can be improved but we see this also encouraging the participants.
Does anyone have any other experience to share or any idea that you think it may work?
Mustafa