A side trip my family and I took in 1999 was a slightly shorter flight but it worked very well for us.
We booked with a consolidator in San Francisco (Skytours) and we were flying San Francisco - Helsinki on Finnair. We also wanted a sidetrip to Barcelona at the beginning of our trip. So it was booked like this:
SFO-HEL 07JUN AY048
HEL-BCN 08JUN AY905
BCN-HEL 15JUN AY906
HEL-SFO 17JUL AY047
It was basically booked as a trip from San Francisco to Barcelona with a 6-week stayover in Finland (which was our main place to visit) Finnair didn't seem to mind, and it worked out to $850 roundtrip per adult.
Very nice
Kai