I don't know why you cannot send me a PM, but in any case I am away travelling and do not get much of a chance to log onto the internet. Currently touring Africa in my MAN camper, returning north ready for the Sahara later this year.
As for your enquiry, I would advise against it. I did it, and would not do so again - it just doesn't seem to be worth the hassle and risk, in CAR at least. I did not arrange a travel permit for S.Sudan in advance - being somewhat naive I did not attribute much significance to it and figured that I could sort it out at the border. As I was travelling at the time I made the decision to cross from West to East Africa via CAR, it would have been very hard for me to arrange anything in advance. At the border I was permitted entry (had a normal Sudan visa issued in Chad, as I did initially plan to go North to Khartoum for a detour via Ethiopia). From what I gathered at the border, my arrival was rather unusual and nobody wanted to see my turn around and backtrack through CAR, with the tales of my experiences there!
I would think hard before deciding to go across CAR to S.Sudan. Also, if you were to pull off this route, consider your vehicle well. 4x4 is essential. Now, as South Sudan is an independent nation I would look into their regulations a bit more and obtaining permission beforehand is probably necessary (probably was then too...)