In Poseray you have a groups tab.
There are all the groups, but sometimes the groups are different from the materials (like in Poser figures).
You can use the Rearrange option to have the model dived in groups by material (Materials -> Groups).
Then you can disable groups by clicking on the icon in front of the group/material name.
Leave 16 materials enabled, and disable the rest, and export as '[object name]-01' or something like that. Then do the same for the next 16 groups and call them '[object name]-02', and so on. That way you have more objects, but smaller in size. If you load those in Terragen and have their origins at the same coordinates, the object should look as one whole.
You might need to run all the small objects through Poseray again (just load and save again with the same filename), because sometimes the other materials are still listed in the mtl file the first time round. Second time that extra info is not available, and doesn't get taken along.
It might take some time, but it has worked for me pretty well in this project:
http://www.terragen.org/gallery/displayimage.php?pos=-5184http://www.terragen.org/index.php?topic=4293.0http://forums.planetside.co.uk/index.php?topic=3751.0