Shawn Kendall at IMI is currently writing a Java3D integrated physics library. When done it should be the easiest path to this rto of thing but hes still in Alpha on it. (Im using it in a project of mine as an Alpha tester.)
What aspect of physics simulation is planned for the IMI project?
I classify physics engine features very broadly as (in increasing complexity):
1. collision avoidance
2. collision detection and response (contrained motion)
3. resting contact (contrained motion with stacking of objects)
4. joints (constrainted contact)
5. foward kinematics
6. reverse kinematics
For what it's worth, Genesis FX currently has level 2 partially complete (interparticle collisions and resting contact coming in v1.1).