In principle the border crossing at Uweinat is closed for Europeans, but some travelers did make it accross to Sudan. I'm not sure about the current status of mandatory escort in Libya though, if that's still in place then it's a no-go.
More importantly, the Kufra - Mellit track is a very demanding route, it needs much imagination to qualify as a piste. It's basically open desert with some ruts in the sand made by trucks. It's a minimum two vehicle route, with no fuel for about 1000 kms.
Can give more details if you like, contact me direct through the FJE website.