Encontrou uma maneira de fazer isso:
server {
listen 80;
server_name name myapp.mycompany.com;
root /opt/apps/myapp_large_name/public;
passenger_enabled on;
client_max_body_size 4M;
location /monit/ {
rewrite ^/monit/(.*)$ /$1 break;
proxy_pass http://localhost:2812;
}
}