I want internal traffic to an internal domain to go out through an external link and then come back in. How can I do that?

You want to do a Loopback NAT? 
You can do a NAT to the external Interface of the firewall and use SNAT as well (MASQ), which will do the trick.

Do: NAT: 

Coming from Client (or ANY) Translated Source: MASQ

Going to: Firewall WAN IP Translate Destination: Server internal

Service ANY

That should do the trick. 

Instead of Service ANY you could only use the service(s) you need to go to this internal machine. Otherwise traffic on every port will be NATed to the internal device.

Instead of Service ANY you could only use the service(s) you need to go to this internal machine. Otherwise traffic on every port will be NATed to the internal device.