Thats true. Site Systems role can only exist on Domain Members.
Anyway this is improved in 2012, as you dont have to switch the entire site infrastructure into the former state called Native mode in 2007 and all Clients require cert.
Now the SCCM 2012 can support both kind of Clients and this makes it easyer, however you still need to make a secure path from the internet to the SCCM server.
The Heavy solution is to create a domain in the perimeter.
The easy way is to either create a Firewall rule that allow https trafic to the MP on the Inside Network MP SCCM Server. Read more here:
http://technet.microsoft.com/en-us/library/dd8eb74e-3490-446e-b328-e67f3e85c779#Support_Internet_Clients
Regarding the TMG beeing discontinued, thats true. But the its some years ahead, and alot can happen until then.
"Forefront Threat Management Gateway 2010 and the product will be discontinued after April 14, 2020. Mainstream support will cease after April 14, 2015."
The other solution is to consider Direct Access.
Hope this was helpfull.
Nicolai