Hallo
I mostly have connections, and I always have access to the lounges also on the domestic part.
SFO-ORD-FRA-DUS access at SFO,ORD,FRA
DUS-FRA-ORD-SFO access at DUS,FRA,ORD
you just have to show the international boarding pass to the loungelady.
The international and the domestic flights must be on the same day!