If it was a BBCode tag then there would still be a post (or news item) visible to all but with no text in it for those not included? That would be .. odd.
Indeed.  But as he mentioned, he wanted to make it a sticky post on the news page.
If he used one of the news posts with the "Welcome Message" at the top, he could easily have something like the following in this message at the top:
| Welcome to Followers of Nobility site! Feel free to poke around and have fun. 
 [onlyrank=Probation]What did you do!?  Why would you go and get yourself on probation?[/onlyrank]
 
 [onlyrank=
 "Not Logged In" - just spitballing hereNLI]Please click "Create Account" in the upper left to get yourself logged in to the site.  You can't exactly do anything if you're not.  And we want to harvest your email addresses to sell to spammers.  That last part was a joke.[/onlyrank] | 
Which would then show the main mesasgef for everyone at the top, then only show the probationary message for probates, and the NLI message for anyone not logged in.
That is, of course, unless he has actual news items (Hogger down!) specific for those ranks, which seems odd to me.  I don't think that's what he's trying to accomplish though.
One of the items on the whiteboard though is "Security Groups/Forum Groups" which are security items denoted by groups of ranks, rather than the current "Rank and Above" method.
Addendum:
In your case, Ennara, you could make a simple "Everyone Read this" sticky, and simply modify which info shows for which ranks. Example:
[onlyranks=Officer,Admin]Click "Admin" on the menu to tinker with the administrative features[/onlyranks]
[onlyranks=Member,Veteran,Officer,Admin]Click on an event on the calendar to sign up for raids[/onlyranks]
[onlyranks=NLI]You need to create an account before you can do anything, noob[/onlyranks]
This would solve the need to have 10 millions stickies (something I'm firmly against).  I could be a single sticky in the "General" tag, with every other tag selected as a secondary tag so that it just shows regardless of the filters the user has selected, and then clicking it would contain information pertinent only to their rank.