We added the notion of "anchor" in last beta http://beta.plantuml.net/plantuml.jar
With this last beta, you can have:
@startuml
!pragma teoz true
{start} Alice -> Bob : start doing things during duration
Bob -> Max : something
Max -> Bob : something else
{end} Bob -> Alice : finish
{start} <-> {end} : some time
@enduml
You define anchors using brackets.
This only works with the new "teoz" rendering engine. We won't make it work with the old default "puma" rendering engine because this one is too old and difficult to change.
Do not use too long text, because this kind of link has no impact on layout, so you can unfortunately have text overmap.
Please post any feedback, thanks!