This feature could be very handy. I would also leverage the biggest strenght of PlantUML - sequence diagrams.
In principle, the communication diagram is another representation of messages between objects. Some people would understand the communication diagram better than sequence.
Practically it would mean extending line descriptions in object diagram.
This would also allow simple transition between diagram - the designer can decide if the same situation is better to be described by sequence or communication diagram. (This was also supported in Rational Rose where a diagram was possible to simply convert to another.)