Ok, that is strange. Now I see the new post come through, and it is before the one Carlos made.
To answer the original question. Technically, no you can't "fool" the portal alias system. Whatever domain is used when the request is made to DNN is what will be used to compare to the portal alias table.
If the one on the Url is not the real one and is masking the one that actually makes the request then you will not get the Portal you are expecting.
What you could do though, is rewrite the Url back using the SiteUrls.config and that could then be mapped to the actual home tab of the parent portal.
But to do this, you would always need either the portalId or the TabId passed along with the domain name.
Another way to do a "parent portal" is to just give it a different Host Name on the same domain. If they will allow multiple host names or any host name (a wild card in DNS) then you can seperate your portals by using :
parent1.mydomain.com, parent2.mydomain.com, etc.