We've fixed some issues in last beta http://beta.plantuml.net/plantuml.jar
There is a not-very-documented syntax for FontColor that allow to automatically switch between two colors depending on the backgroud. We've just extended this for HyperLinkColor so you can now have:
!$FGCOLOR = "black"
!$BGCOLOR = "white"
!$DARK_BG = "#565656"
rectangle "Link to [[google.com Google]]"
And here's a long description about something
that may or may not include a [[google.com cool link somewhere]]
Does it help ?