In the package do you have 'Furniture (other than beds) clicked? Because FFCaptiveMarker is a furniture marker .
I tried this when the package was sandbox, and it did not work. But now they are travel like the normal captives.
Bethesda used a travel package with the FFSCaptiveMarker as the target (using it as a linked reference). Then in the NPC script it checks if they are sitting and sets them restrained so that they don't get back up, and ignorecrime. Have a look at FFSupermutantCaptive for the scripting.
The problem is not they are getting back up, its that they are not sitting down. They simple stand where I place them in the geck, and dont move. I need to get them to sit down in the first place. I have a travel package for each of them, with the marker as its target, so its even more specific than the general FFSupermutantCaptives package that uses the linked ref, which I tried originally, and which did not work either... :/