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My preferred solution would be to add a "To:" field (with a radio button beside it) in Message Inbox so you can use the "Reply text" field to send private messages that aren't replies1.
This is better than the other solutions because it has zero risk of exposing your chatter to public view. All of the other proposals handle a few specific typo cases but still leave plenty of room for embarrassment: you might leave off "msg ", "/", or both, mispell "msg", use "\" instead of "/", insert or leave of a space, etc. And some of those cases are impossible to trap (while many are annoying to trap -- to me all of them are).
It even prevents misdelivery when you leave off the monkname (I know and has gotten some very juicy private messages by mistake) or forget to deal with the spaces in a monkname. You could still typo the monkname and send to the wrong person, but getting a typo that matches a different active monkname is very unlikely.
It also has the advantage of allowing you to send full-length messages instead of messages shorter than $maxmsglen - length("/msg ") - length($monkname).
Thanks to PodMaster for reminding me that I wanted to implement this (and sorry I didn't see your question until after you'd made that patch).
1 So that field will probably be relabeled.- tye
In reply to Re: RFE against certain Humiliating situations (better)