Hi Thomas,
Yes, you can get any entity's bounding box in SIGVerse, by using SIGViewer.
Please Go to "Option" on the top, and click it. Then click "Dynamics View". You can now see, the bounding box of your 3d model in green color.
If you click "Dynamics View" again, then you can see both the original visualization of your model and bounding box of your 3d model in green color, together.
And yes, you are right, the position of bounding box refers to the center of the box. But in advanced cases, if you create your own model using OpenHRP format, then center of your bounding box will be the position of your JOINT0.
Hope that helps.
Best Regards,
Raghav