Hi, 
Those are all good with forcing a management point.  Unfortunately (for me anyway), the Management point is not an issue as all clients in Domain B are registered with the desired MP.,   
  
  
What I have is  
  
Domain A 
CAS 
Primary Site Server  
Management Point 
DP 
SUP 
DB 
  
Domain B 
Additional Site Server 
Management Point 
DP 
SUP 
  
I have Point to Point firewall rules between the 2 servers and comms is working well.  The Clients in Domain B are registered on the site and have picked up the server in Domain B as the management point 
  
What I am seeing is that the Software Update source server is coming in as the primary site server in Domain A.  I am also seeing that the Software update point is not Synchronising from a source within the SCCM environment, where the Primary site is syncing from the CAS, I have a blue exclamation and the Sync Source is listed as Microsoft (WSUS on server in Domain B is configured as full WSUS prior to SCCM SUP install) 
  
  
What I cannot do is 
  
1. Establish Domain trust between the domains - IE no Secondary site within the domain 
2. Open the firewall from all the servers in Domain B to the Primary site server in Domain A - just not even politically viable