Hello Marcos,
"host of web disp.domain" and "mysite.domain" should point to the same server, where the Web Dispatcher is running at.
Otherwise, how would you expect that the request going to "mysite.domain" would reach the Web Dispatcher? Note that your end users only need to connect to the Web Dispatcher. They don't need to have direct access to the system behind the Web Disp.
And changing the Web Disp. ports will not interfere with the redirect behavior, as long as you adjust the "PORT" option of the redirect parameter accordantly.
Regards,
Isaías