Thomas:
On your 520:
access-list permit tcp any 12.19.224.xx eq www
access-group in interface outside
static (dmz,outside) 172.16.0.21 12.19.224.xx netmask 255.255.255.255
This will configure a static NAT for the DMZ web server to the outside, and acl entries to permit port 80 access.
At the remote sites users should now be able to access your DMZ web server on 12.19.224.xx on whatever ports you've opened with the access-list.
That should do it!
Rich