Hi Jorge,
At this point, a dynamic or wildcard ForceDomain is not possibe. We do have plans for making the Force settings more dynamic and you can view an open tracker entry on that subject
here
.
The most simple method would be to deploy two applications, one for use outside of the proxy, using the ForceDomain settings, and another one for non-proxy use, and which would not have the ForceDomain setting. These two applications would be identical, except for the ForceDomain setting.
Deploying would be a bit more work, as you would have to deploy both of them. There is another option you may want to look into, using Location settings within your web.config. This option is discussed in this article here with links to the relevant MSDN articles on the subject.
Hope this gives you some ideas,
Palli