I mentioned in the other topic as well that this is already possible without using a plugin if you set the moderator group’s ’who can message this group’ setting to ‘everyone’ then it allows even those people who aren’t in a PM allowed group to PM the moderators group.
However, it is a little obscure to set up as staff, and the message button itself appears on the moderator group page which may not be the easiest place to find it.
New users are generally only at TL0 for around 10 or so minutes after reading a handful of topics, so it may be too brief a window of time for many to want to PM a moderator (possibly why this isn’t requested more). Though it could be more useful to sites with more restricted PM thresholds, or where it’s disabled completely.