Nginx – Wildcard records with Nginx to handle custom domains

Here's my 4, going on 5 hour problem:

I've set up a WordPress multisite instance that's going to be handling sites at domain.com, subdomain.domain.com and customdomain.com. There will be N number of sites using customdomain.com, so I'd prefer not creating records for each. On the server, I have Nginx in front of Apache.

What I'd like to do is set up a wildcard record in Nginx to handle all of the custom domains. Right now, it looks something like this:

server {
    listen 80;
    server_name _;
    root /home/server_user/web/production;
    client_max_body_size 50M;
    client_body_buffer_size 128k;

    location / {
        access_log      off;
        proxy_pass http://localhost:8080;
        proxy_set_header X-Real-IP  $remote_addr;
        proxy_set_header Host $proxy_host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }

}

With this setup, it will pass requests to Apache and serve the dynamic content but returns 404s for all of the static content. If I change 'server_name' to 'customdomain.com', Nginx starts serving static content again. When I change 'server_name' to '_' or any other catch-all pattern, Nginx falls on its face.

Any ideas?