It appears that key usage information is available as attributes of each menu item:
tell application "System Events"
get name of menu item 2 of menu 3 of menu bar 1 of process "Finder"
--> "New Folder"
get every attribute of menu item 2 of menu 3 of menu bar 1 of process "Finder"
--> {attribute "AXRole" of menu item "New Finder Window" of menu "File" of menu bar item "File" of menu bar 1 of application process "Finder", [...]
get properties of attribute "AXMenuItemCmdChar" of [...]
--> {value:"N", class:attribute, settable:false, name:"AXMenuItemCmdChar"}
get properties of attribute "AXMenuItemCmdModifiers" of [...]
--> {value:1, class:attribute, settable:false, name:"AXMenuItemCmdModifiers"}
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