Writing a custom post page is probably the way to go.
It’s actually fairly easy to write a from-scratch “post form” (see my plugin http://cjbehm.dyndns.org/wingingit/quick-post/ for an example). Your post form can call the wp-admin/post.php page with appropriately named fields and it will do the right thing.
Plus, if you add a hook into the publish_post function, you can redirect back to wherever you want (such as the REFERER page, which is what I do).
Thanks for the tip! I actually ran across your quick-post plugin, but hadn’t looked carefully enough at it. I like what you’ve done and I think I’ll first try just making a special page that uses your quick-post plugin so that registered users can submit their own simple posting, and perhaps I’ll add a couple more fields and helpful text so people know what they should post. I like how you have the rest of the fields hidden, but it still uses all the built-in posting codes.