Hi Alex,
I didn't see a response here so I thought I would jump in. Anonymous posts are not currently allowed in Discussions in Haiku. That said, there are a couple of ways you may be able to facilitate such a conversation.
Creating a whole class Wikiproject would allow students to post text blocks. While not exactly a discussion format, the posts would be anonymous. The downside here is that it does allow all students to edit each others posts. However, with appropriate instructions and monitoring, it could create a nice anonymous discussion space.
If you do not need a group or whole-class format, you could also create a one-to-one discussion in which the teacher would be the only one to see the posts. While not anonymous, posts to these discussions could be treated as such. There is also the option of creating an anonymous Poll. Though not facilitating a group discussion, they do allow for anonymous responses.
I hope one of these solution is helpful. I would also encourage you to post or vote for (if it's already there) this idea in our Feedback Forum at
http://feedback.haikulearning.com.
Best,
Jeff