A while ago, I implemented the generic spring and motor constraint, and planned to make a simpler spring constraint 'later'.
Since I found some time today and the generic spring is not very user-friendly, I wrote a patch for the implementation.
I created new functions in die RigidBody API which internally use the functions from the 6dof and 6dof spring constraint, since there is currently no simple spring constraint in bullet (to my knowledge). This should make using an native bullet simple spring constraint easier if bullet comes up with one in the future (I'd say a simple spring would give more performance).
The spring constraint locks all rotational axes and the linear X and Y axis, and uses the Z axis as the springs axis.