Try excluding requests with a certain referer:
RewriteEngine On
RewriteBase /
RewriteCond %{REMOTE_ADDR} !=10.10.10.10
RewriteCond %{REQUEST_URI} !^/soon\.php$
RewriteCond %{HTTP_REFERER} !soon\.php
RewriteRule ^(.*)$ http://example.com/soon.php [R=307,L]
Or, just exclude the /soon/
directory:
RewriteEngine On
RewriteBase /
RewriteCond %{REMOTE_ADDR} !=10.10.10.10
RewriteCond %{REQUEST_URI} !^/soon\.php$
RewriteCond %{REQUEST_URI} !^/soon/
RewriteRule ^(.*)$ http://example.com/soon.php [R=307,L]