A couple thoughts on this:
1. Yes. Your website is mostly in your database, so you can backup all your WP files on your local drive using an FTP client, delete them from your root folder and copy them back again when you are ready to resume using WordPress. As long as you don't touch your database everything will work like it did before.
2. You could also just rename your index.php file and load your script into the same folder. It is highly unlikely that any of the other application's files will have the same names as your wp files and folders. (but backup just in case)
3. Why not use a subfloder to test your forum?
4. You are going to make a mess of SEO for your site if you do this, just and FYI :0)
Hope that helps