Run additional service on domain with Wordpress Multisite?

I've got three parked domains configured to use my WordPress Multisite installation. No problems with the configurations there. I can serve up three independent WordPress blogs at site1.com, site2.com, site3.com. All WordPress files are located in my web root at /public_html.

What I'd like to do is add a completely separate service to one of my sites. For example, let's say I want to run the Foo Service that I've installed in a subdirectory of my web root (public_html/foo_service), and I want visitors to be able to browse to site1.com/foo in order to access that service. How do I "escape" the WordPress configuration? Do I configure a different site within my WordPress network? Can I add some redirect to my root .htaccess?

The way my .htaccess is configured now, it would appear that anything I append to my URL is routed through WordPress' index.php.

Here's my current .htaccess:

SetEnv PHPRC /home/user123/public_html/php.ini
RewriteEngine On
RewriteCond %{HTTPS} !on
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}

RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]