I had a similar need to make opaque objects transparent when editing scenes.
My solution was to detatch the object and re-attach with the correct RenderQueue mode ( opaque or transparent ). Be sure to do this in within an iteration of the scenegraph update cycle. You will need to update the parent ( updateGeometric***** ) after modyfying, so it carries a little cost - as long as this happens no more than once every few seconds it will be fine.
there is a small test case which uses transparency in this thread: http://www.jmonkeyengine.com/jmeforum/index.php?topic=9579.msg73890#msg73890
just replace Alphastate with Blendstate if you use jme2