Hope this is of assistance and helpful to support your situation and are considering, the option does sound technically feasible and do able, please consider the below:
+ Site to Site Communication
+ Site Server <-- to -- > Site System , By default, communication between the site server and site systems is bi- port 445
+ Site Server <-- to --> Sofware update point, ports 445, (80 or 8530) and (443 or (8531)
+ Client --> Software Update Point communication ((80 or 8530) and (443 or (8531),
+ if these ports map out well on your hierachy, it should be possible.
+ When your hierarchy contains a central administration site, install a software update point at this site that synchronizes with Windows Server Update Services (WSUS) before you install a software update point at any child primary site. When you install
software update points at a child primary site, configure it to synchronize with the software update point at the central administration site.
Do consider links to:
Planning for Sites and Hierarchies in Configuration Manager:
http://technet.microsoft.com/en-us/library/gg712681.aspx
Planning for Site Systems in Configuration Manager :
http://technet.microsoft.com/en-us/library/64aa34e1-c465-4eb8-820b-5c1702ab55ae#Plan_Where_to_Install_Sites
Planning for Software Updates in Configuration Manager:
http://technet.microsoft.com/en-us/library/gg712696.aspx
Prequisites for Software Updates in Configuration Manager :
http://technet.microsoft.com/en-us/library/hh237372.aspx
Ihopei have not missed anything to take into consideration and planning.