blogs.dir & manual uploading content

Your code/plugin can be multisite AND single WP compatible with the following I figure.

$uploads = wp_upload_dir(); //Multisite or not, you get the correct upload folder array

http://codex.wordpress.org/Function_Reference/wp_upload_dir

print_r($uploads); //note differences with codex when in a Multisite, blogs.dir shows up?

Can I add directories and content using the blogs.dir path: wp-content/blogs.dir/%BLOG_ID%/files/ ?

So the the following should do what you seek:

$uploads = wp_upload_dir();
$path = $uploads['basedir'] .  '/gallery/';
wp_mkdir_p( $path , 0755 );

http://codex.wordpress.org/Function_Reference/wp_mkdir_p