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The script mev_battle_attack_select will build this menu (listing only the targets that are valid depending on the move), and it creates an array "my_target" in the menu instance, which the menu later uses to slate the action when you confirm. Each entry in this array is the instance ID of a battle monster object, so you could just copy it (after it's been completely built) to your cursor object and then get positions etc by going through the instance IDs.

Now, your main question is how to keep this in sync with what you're selecting in the menu. It's pretty simple: script_step. Menu objects will execute this script after their regular menu-handing behavior if it's not NONE. (There's a whole bunch of these customizable behavior scripts for more complicated menus - check parent_menu's create event - but most of the time we only change script_draw)

Make a script that tells the battle cursor what the currently highlighted item in the menu is, and in mev_battle_attack_select, set the script_step variable in the menu to this script. (Heck, you could forgo the array-in-cursor entirely and just tell it to focus on an instance ID by having the menu check the array... that's probably easier?)

Also, you could use this to make the cursor automatically get destroyed when the menu is closed: in its create event and in this script, set an alarm to 2 steps. When the alarm happens, instance_destroy(). As long as the target menu exists, it will keep resetting the alarm before it can trigger, but when it's destroyed (after you confirm the target or cancel out), the alarm will run out and also destroy the target cursor.