First suggestion from me will be: try handling corresponding requests by
newly created "HighlightConnectionTargetsEditPolicy". See generated ???BaseItemSemanticEditPolicy.getSemanticCommand()
method as a prototype of request handling. I think you should handle "CreateRelationshipRequest".
You can choose different ways to implement this feature (+ install this EditPolicy
onto the different diagram elements in accordance), but i think first you
can try installing this EditPolicy onto the EditPart representing link source
diagram element and highlight all the possible link targets from there on
link creation start. The open question is how to remove these "highligh frames"
(from wich place in a code).