Getting radical realtime transparency in a project can be slow and frustrating, especially in the beginning. Most folks don't know this, but in order to have public conversations, leaders need to send out a ridiculous number of private messages to get things rolling. In fact, looking at my own inbox history for the past half-decade, I've sent anywhere between 2-20 private messages - on average (not maximum, average) - to get a single public message during the early stages of a project's "open" life.
You really need to keep poking people in private asking them to put their messages public. It's thankless and invisible work. It takes a while to build a new cultural habit, and for a while it's going to seem like you'll be doing this forever... but trust me, it will come. It's going to take longer than you want it to, it's going to take an unexpected route, but keep the faith - it will come.
There are three strategies it's useful to have up your sleeve for times like this.
Start the conversation in private, then say something like "hey, this is really good, could you resend it to the public list and I'll reply there?" This is good for starters if folks are new to the "default to open" concept and are reacting with great nervousness. This nervousness stems from wariness that they may not want to go public with some hypothetical future thing - in effect, worrying about a problem that hasn't happened yet. Going this route allows beginners in radical transparency to look at something they've already written and assess the risk for only that specific situation - no unknowns here, no future commitments. After a few times of going "oh, I guess that retroactive transparency was okay!" it's much easier to ask people to give "open by default" a chance.
Publicly announce that you'll only respond to things sent to the public list. Reply to private emails with a reminder of this. This only works only if the people you're trying to persuade are unable to route around you. It's also a bit of a strongarm tactic, not appropriate for all situations and best used in moderation if at all. But if you're a project manager, or an instructor, or a senior engineer, or something of the sort, you might be able to get away with it - and boy, folks learn fast this way.
Get others to help you with the nudges-to-public. Those 20 private emails to get a single public email? No reason why you've got to be the only one doing it. Train others to become Agents of Transparency as soon as you can, especially if they were once on the other side of the conversation. To begin with, ask them to work specific mailing lists, specific people, or specific conversation threads into the public eye - coach them from behind if needed. After a little while, they'll be able to do it on their own - then just ask them to keep an eye out in general, and hey presto!
The key thing to keep in mind is that this is an investment. You're putting resources into something that may not see returns for a little while. But the returns will come, and they'll be worth it - when a project tips over into living, breathing, and practicing true realtime transparency, the results of the culture shift can be stunningly refreshing.
Keep going.