Renaming wp-admin without hard-coding it. Is it really possible?

For a fully-working solution, you will need to modify certain hard-coded urls in core files, which is not fun to have to do each time WP is upgraded.

One easier option is to protect the wp-admin folder with an .htaccess file. You could basically deny all IP addresses and whitelist the few IP’s that you or your editors might use. You could also use HTTP AUTH to add another layer of password protection. It will prevent unauthenticated users from viewing the wp-login page.

And regarding renaming your folder, I’m not sure that it will really make your site more secure. For example, if you use any AJAX plugins, a hacker will know your admin folder location right away! And there are other simple ways to determine it as well…