IIRC, during Hanse Davion's wedding on Terra (as told in Michael Stackpole's Warrior triology), someone (maybe Morgan Hansek-Davion?) snuck into an underground ComStar facility and saw rows and rows of Star League era 'Mechs.
So, I would guess that Star League era 'Mechs would have been available to ComStar units even during the SSW, but they might not have used them in case they were captured by House units. They might have used them on Tukkyyd.