It's a big job no matter how you do it. I think I'd try to use Articles for that, and put each user's posts in a separate folder. You can limit the user to his or her own folder by making all other resources hidden in the tree.
You'll also want to customize the Manager to limit the user's actions and to provide a simple interface.
NewsPublisher would be simpler to set up, but you'd be missing a lot of the power of Articles for things like creating archives, using tags, and allowing comments.
Here's something else you should probably consider. I've never used it, but you could probably theme it to look like the MODX site and just link to it:
http://codex.wordpress.org/WordPress_MU
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODX info for everyone:
http://bobsguides.com/modx.html
[ed. note: BobRay last edited this post 14 years, 6 months ago.]