Do you just want to grab some parts of the file and display only those, or do you want to stream the file from the server while the camera moves? Anyway, it should be possible with jPCT but the later case will require some streaming on a per object/block base and it may limit octree usage. Or in other words: It's not an easy thing to do, i think.
The former case is much easier. Load your data using with your own loader, maybe setup an octree for it, place the camera and some lights and you should be done (well...almost... :wink: ). A VERY basic example of a landscape can be found in the example-dir that comes with jPCT.
Keep in mind that you are limited to the software renderer when using jPCT in an applet. This is due to the design of LWJGL (the OpenGL binding jPCT uses) and the applet's sandbox.