I don't think mIRC will natively support changing the people who are in the nicklist. There are various security issues possible in such a situation. Of course, custom nicklists also have the same possible problems with users being hidden or changed without a user's knowledge, but a script can be seen and removed, while changes in mIRC may not be as noticeable. Anyhow, the suggestion of being able to hide users in the nicklist (same idea, but only dealing with the one part of it) has been suggested in the past and nothing has come of it, so I don't really expect to see any change.
A custom nicklist is your best option and you should be able to script the same functionality for nicks in the channel as already exists, depending on what you want to do.
Anyhow, we usually aren't told what new features there will be in mIRC until the release date, so your guess is as good as mine, but I just thought I'd comment on it.