Have not heard that it is not allowed to enter Xe Xap NBCA but know that access is difficult due to its location. The main access is by the unpaved road leading from Salavan to Ta-Oy (now partly asphalted for about 30 Km. from Salavan) and further east to Samuoy. This road runs along the northern side of Xe Sap NBCA, but it does not pass through the NBCA area. From this road, small access tracks lead to villages situated on the northern periphery and inside the northern part of the NBCA. Two friends of me were there recently. They said the road conditions were bad but that they could get in but both (on different occasions) were accompanied by government people. From the south-east the best way to enter is by boat on the Xe Kong, Xe Sap and Xe Lon, or by walking.
David/Mai (2Up Chiang Khong) went there early this year (see https://www.gt-rider.com/thailand-motorcycle-forum/part-three-2up-off-road-northern-laos-to-southern-laos-t7337.html) and they probably have better and more up to date information on that area.
Another option is to contact Mr. Bounthong Outensakda who is the manager of the Xe Xap NBCA at the Provincial Agricultural and Forestry Office (PAFO) of
Salavan. Tel: 034 – 211 068
PLEASE DO NOT venture off the main track that runs between Nong and Ta Oi - there are so many unexploded bombs, land mines and bombies that it makes this area one of the most dangerous to go into in all of Laos. But first - to answer your question directly.
As Auke says, we went past this area this last riding season when on our way south from Nong to Ta Oi riding our DRZ400 2Up. We saw a few very small dirt/rock tracks going east of our route and into the NPA. We went down a couple of them just to see what was there - jungle and more jungle, and saw no one. So we assumed there were no restriction problems if we had wanted to ride deeper into this area.
BUT - back to the bombs. Shortly after taking a couple of the side tracks into the NPA and about thirty more minutes to go to get to Ta Oi we came across a mine clearing group - two of them actualy and working side by side. We stopped and talked with a lady that was part of the clearing team and she told us that just the week previous a local was killed by walking only one meter off the main track! And then when we got to Ta Oi, we saw a 500 pound bomb, that just the week before had been found laying in the middle of the river and in the exact same place where the children played and swam every day. The mine clearing team had been called into to defuse it. You can see a picture of me sitting on this defused bomb in our trip report for this area.
So there are no restrictions to venturing into this area that we are aware of, except for all the unexploded ordinace that is to be found everywhere - including just along side the main track that people have been traveling on ever since the end of the war. If we were to ride this track again - no, we would not have gone down a couple of the side tracks as we did.
Feel free to give me a call at 081-9902096 if you want more info.