Clean URL main site, WP outside webroot

I am trying to get a WP install working under a very specific setup. My website root will not be a WP page, but will have WP elements. WP will be accessible through a /blog/ URL. However, here is the problem.

I run everything through a main index.php in my webroot, and keep all other PHP files, including WP, outside the webroot directory. The main site (as well as WP) uses clean URL’s. My site is hosted on a 3rd party cloud system, so I do not have access to the httpd.conf file, where one would normally put aliases and so forth, so I am restricted to .htaccess per-directory files.

It is possible to use a rewrite rule to redirect all /blog/ requests over to another directory. However, the directory must be inside webroot, so that method is out of the question. I have also tried handling the /blog/ requests inside the webroot index.php by directly including the WP index.php fie, but WP does not seem to like that very well – it doesn’t display anything.

So my question is, is there any way to run WP entirely outside of webroot, using clean URL’s on the main site which also incorporates WP elements, without being able to make aliases?