Currently the multilabel segmentation plugin does use a custom widget for offering 3D and 2D interpolation capabilities. These do not however work.
It might have worked on an old branch, but the relevant code does not seem to have had major changes since bug-15696-multilabel-v2014-03-integration and I suspect it did not work then either. A fix for the 3D interpolation can be found at personal/goch/MultilabelSegmentation-interpolation-replacement, however this is basically a patch job that replaces the surfacebasedinterpolationcontroller implemented by @ordas with the our own surfaceinterpolationcontroller. The same is not as straightforward with the slicebasedinterpolation vs the sliceinterpolation as our version has a higher interdependency of UI and code. Additionally the segmentation is a QmitkFunctionality while the multilabelsegmentation is a QmitkAbstractView so keep the migration guide in mind when trying to port functionality.
We should probably consider using the switch to the multilabel plugin as a chance to do a proper redesign of the interpolation code within the frame of an introductory project.