Not the REST Service is forced to run on port 80, its the outspark_vc71.dll which is
attached to character and login service and forced to send any request to stubedore.t:80!
You can use

modified .dll to let it run on port 8900.
No need to modify your (windows) hosts, no need to modify your http service.
Also no need to open your firewall for this port (8900) if you run all fiesta related
services on the same machine.
Simply change this .dll with yours, open your *RestServerSettings*.shn and make sure port 8900
(and also ip 127.0.0.1) is set there.
Exactly for that i modified that ****, nothing else was made there.
For more information read

(german) post.