It would be useful if the next incarnation of MT, if a couple new tags could be created
mt:IfRegistered
mt:IfSignedIn
With these two new tags, specific web page content could be provided which targets ONLY visitors who are either or both, Registered and Signed In.
Seemed like such an obvious thing, but, after an hour or so of searching tags to tips and tricks, it doesn't appear this capacity exists EXCEPT in the Comments area specifically anchored to whether commenting is permitted or not.
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Reported on Movable Type 4.2
It's obvious in terms of being a good feature, but not obvious in how to do it right in a way that keeps Movable Type true to being agnostic toward output. Those tags would work in PHP, JSP, ASP and ColdFusion, but it would require more infrastructure with plain HTML since it'd probably to be implemented as some sort of AJAX system.
The functionality you are talking about is actually not difficult to implement in PHP or JSP if you're already writing your content as PHP or JSP files. If you open your blog's cookies in Firefox's cookie viewer, you'll see that they contain enough information to let you identify a registered user to PHP, JSP or ASP code.