This is a feature request and I am guessing that I am not alone on this:
I have a server that creates a multisite beneath a FQDN which is already in use by another webserver.
Obviously this poses a number of interesting issues many of which are easily solved by DNS manipulation. Network administration being one issue which can be solved this way.
But user registration is out of the question because inevitably you will be redirected to network_site_url which is not a WordPress site at all - its not even in the same physical location.
Could we please have the user registration and logon not dependent on redirection to the main site?