Actually, this is already supported. However, to achieve the effect you need a preparation step prior to running the edge router.
Use the Tools > Constraints > Port Constraints... tool to specify the "port constraints" that the edge router should keep. For example, choose:
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Configuration Method: "From Sketch"
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Fix Source Port: enabled
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Fix Target Port: enabled
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Act on: "All Ports"
and click OK in that dialog, then all ports of your diagram will get tagged appropriately, and on the subsequent run of the edge router it will honor the tags and keep the existing port coordinates.
Note that this is also true for any other layout algorithm that supports port constraints (e.g., Layout > Hierarchical...) and that you run after having specified the port constraints.
Note also that all your ports remain tagged until you specify other port constraints or "Remove all Port Constraints" through the tool's dialog.
