1. http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuKhKjKkoKYX9BIxXoiilILKeoapFAE5ApKjH076v780ie380
2. http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuKhKjKkoKYX9BIxXoiilILKeoapFAB7YSaZDIm5A0m00
i guess what you are proposing is not possible in the old preprocessor (but I'd like to have it for the old preprocessor if possible).
For preprocessorV2 it looks it works fine:
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuL9CAyrBBidCprF8ooz9rIX9hIZHv5BCJ5C0CnKKRHMKb3IvG78AHPdf6ILM864knTISubGWcPUIcGQcOAPmSINgsYLPvHQd6u8rQIW1IONKt8AIIYLDJ4cGc0DGEz0b0000
(--------)