Mussala is higher than the pistes of Borovets, and they plan only 3 days of snow there (40-60%) in the next 2 weeks. It seems that it will be difficult to estimate if it s enough or not to open the pistes...
I think you won t have problems if you are beginners because the small runs will be covered with artificial snow.
But for intermediate and advanced skiers/boarders coming before New Year in every Bulgarian resort, your only hope is a ritual snow dance...