User first tab into the overview area which gets the focus. Then I want the user to be able to use the arrow keys to replace the mouse. I would then translate this into Diagram.position call, similar to how DrawCommandHandler allows you to move a node, with Ctrl key being a finer displacement.
OK, so you don’t want to use the PanningTool at all. You just want to handle the arrow keys in the Overview.
Just as with a regular Diagram, override its doKeyDown method. An example is shown in GoJS Commands -- Northwoods Software. For each call with a particular key, such as e.key === "Up", call the Diagram.scroll method on the Overview.observed Diagram.