For those tasks, milestones, or external inputs where the date is still unknown, leave those line unbaselined. If the rest of your project is baselined, then the dates for the unknown lines will be blank.
EDIT: A baseline is more than a snapshot. It serves as your commitment and an input for measurement using things like EV.
Once baselined, I do not think you can reverse it. You can purposely baseline a select portion of your project, leaving control accounts and work packages unbaselined, which is essentially progressive elaboration, which is what you want. When your dates become more certain, you can strike a baseline for those accounts or packages or milestones and begin working towards them.
MSProject allows for multiple baselines, as well. Or, you can rebaseline if a date changed and was approved through your change control processes.
The whole point behind a baseline is that it does not arbitrarily change. Once set, it is set and you measure from there.
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