Why don’t you just put a GoPort at that point? You can change the appearance and size as you wish – even making it not Visible.
This is actually how GoToolLinking works – while the user is dragging around the end of the link, the user is actually just moving a temporary port that the link is now connected to.
Well, then, make that GoPort Selectable, so the user can select it and drag it. You might want to set Copyable, Deletable, IsValidFrom, and IsValidTo to false.
Are you trying to let the user extend the path of a link by moving an end? Yes, you could implement something like this. This reminds me of the RoutingLinkTool in Demo1, except that you want to use a similar tool when “relinking”, not when drawing a “new link”.