Using WP for Index Page of External Site

My company has been using WP for a subdomain blog for our main site. They’ve decided that they’d like to replace the index of the site with the WP blog. This is quite easy to do, but they want users to have the ability login to our site from the blog (possibly with a lightbox).

Are there any existing best practices, examples, or plugins of anyone doing this? I’ve started welding some things together and realized I’d need to roll my own CSRF solution. Any help is greatly appreciated.