Is it possible to POST to an action if the system is offline? There are easier ways to do this for sure, but... 

I have the Mailchimp Subscribe plugin installed to build our email list etc... and it works perfectly!

However, we are now building a "Coming Soon" page with the site turned off to the public while we build out the site. 

Is it possible to accept a certain action while the system is offline so that the POST request is accepted and doesnt return a 503 error?

Again, this is possible through using just a Mailchimp form itself but I'm interested to see if this sort of thing is possible if I ever needed this sort of control in the future.
Completely untested, but something like this in your craft/config/general.php file should work:

return array(
   'isSystemOn' =&gt; (isset($_REQUEST['action']) &amp;&amp; !$_REQUEST['action'] != 'mailchimpSubscribe/list/Subscribe'),
);