Sure, just use a GoLabeledLink, and have the GoLabeledLink.MidLabel be a GoPort with whatever appearance and behavior you like.
In fact the FamilyTree sample application demonstrates this, although the effect is not visible in the application.
It is more obviously used in the Fishbone sample application, although even there it is not as obviously connecting a link to a specific port located in the middle of another link, due to some more sophisticated positioning of the end of the link rather than the default behavior.
The MidLabel need not even be a GoText or a GoPort -- it can be any GoGroup or even a complicated GoNode.
Furthermore, you needn't be restricted to just using the MidLabel property of a GoLabeledLink. You can have as many child objects as you want, if you are willing to override GoLabeledLink.LayoutChildren to position those children where you like. An example of this is in the Processor sample application, where the FlowLink class supports as many LinkLabels as you want, each independently movable by the user.