You still need to stop the existing web server to get spark to take over port 80, and restart it when you are done, so it's not quite that simple.
Also, it's not hard to keep a maintenance conf file around, pointing to a different directory. At least, it's certainly no harder than downloading a second web server. Then it's just "sudo ln /etc/sites-available/maintenance /etc/sites-enabled/conf"
<h1>Maintenance</h1><p>will be back soon!</p>
Oh. It's written in C. Got it.