Back again with a little more time...
The last time I was in CAR is one year ago. We came from Yaounde/Cameroon and went to Bayanga for Gorillatrekking. This road is frequently used by Timbertrucks (corrugations!). We left the CAR via Garoua-Boulai, so we passed the Diamond-Area near Carnot. This was no problem at all.
If you enter from there you will find tarmac at Bossembele up to Sibut. You might leave the CAR in every big settlement like Bangassou, Rafai, Zemio by piroge to the RDC. Stay at the missions there and you will get good informations of the further situation. Find the missions here here
. If you decide to go directly to South Sudan, keep an eye on the part from Obo to Bambouti. By bike it might be easier, but you will find a lot of trial-passages there. If you enter Uganda don't go to Gulu. Arua is safe, from there go via Pakwach to Kampala.
If you decide to enter RDC imho it would be the best way to go to Aru and then Arua.
At the moment there are two spanish cars on this way.
Keep in mind that the rainy-season starts again in the end of January, it would be a good idea to be in Maridi up to than.
Hope I could help you.