Plugins hosted at WordPress.org need to have a file called readme.txt
that's formatted in a particular way. See this: http://wordpress.org/extend/plugins/about/readme.txt
One of the tags near the top of readme.txt is Stable tag:
.
When a Wordpress.org blog administrator goes to download a plugin, wordpress.org looks in svn at trunk/readme.txt, grabs the stable tag, then gets the plugin from tags/stable-tag/
So developers releasing updates need to create a new stable tag, then change the readme.txt and check it into the trunk.
I don't know if you can pull off this behavior with svn:externals. But it's worth a try.