Is that currently possible?
Please see the example:
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuU9ApiyjoCzBpIjHK08n7PKK4eiLm2o5QgvQBgwy_99Ka92V8PS246Ggb2S2rPPaP1OhvES1wLI5eipqZ1AkrBmK1P1Q91r6k7JafAOXQN00ie1E0G00
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I think the only way you can achieve this is with hidden connections.
e.g. http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuKhEpot8pqlDAr5G0Z4TbHGIYnN0B8LghbekhhpyabIGa9yXbm8GP2gK9mBLbcHa5Ylavm7fL8MYpFIC4gxKl1G5a5ea7KQuTEIafY5hWDYc6vsHcPAIcXSBDayNQ26w14YRBiXO1w1UeDi1