I see lots of maintenance page solutions but I would like to keep serving
images and a few directories. Is that possible?

Right now we have:
if (-f $document_root/maintenance.php) {
rewrite ^(.*)$ /maintenance.php last;
break;
}

But we would like to make the privacypolicy.php page and the images & promo
directories always available. That way people can always see our
advertisers, images, and privacy policy.