I was curious when I saw this thread, so I added the Outliner in my projcet, and I just tested it on JME 3.2 and it works.
I did find that using SelectObjectOutliner.OUTLINER_TYPE_FILTER works but using SelectObjectOutliner.OUTLINER_TYPE_MATERIAL did not work.
I was able to fix the Material mode by attaching the new wireframe geometry to the target spatial, rather than attaching it to the rootNode as it was previously coded.
Also thanks you @polinc for the very useful library! 
I like using filter mode for outlining friendly players, since the Filter mode continues to show the outline even when the the outline object is behind other spatials and out of line-of-sight.
Whereas the material mode is useful for objects that need the outline to abide by the usual scene depth.
If you’re using FilterMode on 3.2 then it should be working. If not, then my best guess would be that youur filterPostProcessor is not setup correctly, or you’re passing in bad/null references to assetManager and renderManager.