OK, I went to the shop at Wynne Volvo over lunch--we checked both their quick reference specification book and their on-line system VADI (?) and we couldn't find a specific reference for torque values for the 1999 s80 cam cover/housing. We also looked at the specs for other vehicles using that same basic engine (XC90, others) and got the same result: nada.
I talked to their lead technician (certified Volvo master tech) and he thought the cam cover was held in place with "around 40" M7 bolts and that they should be tightened to 17NM (about 12.5 foot-lbs). This figure fits with Volvo's rule of thumb for torquing an M7 bolt (grade 8.8).
FWIW, their spec on an M6 bolt is 10NM; M8 is tightened to 24NM.
Sorry I couldn't find anything definite.
Steve A-