Please note that this functionality is already inherently available via using a custom mod_rewrite rewrite rule.
[1] Create and customize a file called: maintenance.php
[2] Add the following to your .htaccess
Code:
RewriteEngine On
RewriteRule !^maintenance.php$ /maintenance.php [R=302,L]
Any accesses to any page on the site OTHER than maintenance.php will be redirected to maintenance.php. Therefore, this disables all links/pages/etc. on the site other than maintenance.php
You can then comment out that rewrite rule as desired to enable/disable maintenance mode within seconds.
A full on feature to automate this, while not out of the question, seems a bit heavy handed given the 2-step process to activate it as it is now. In addition, many scripts in existence already have inherent "Maintenance Mode" functions within them.
EDIT
To address monarobase's request to allow maintenance mode to "exempt" certain IPs, you could modify Step 2 to the following rule:
Code:
RewriteEngine On
RewriteCond %{REMOTE_ADDR} !^123\.123\.123\.123$
RewriteCond %{REMOTE_ADDR} !^100\.100\.
RewriteRule !^maintenance.php$ /maintenance.php [R=302,L]
And so on. The above would exempt "123.123.123.123" and "100.100.*.*" from the rewrite and thus allow them to access the site. This admittedly can be a bit more complex since you need some basic Regular Expressions knowledge, but is still readily possible with cPanel as it stands right now.