I am not sure if this will help but you can actually direct windows to disable assigning a new comport number to a particular device using their vid and pid number, you just need to edit a registry assuming all your devices under test does have the same PID and VID.
Go to HKLM\system\currentcontrolset\control\usbflags\ then create a new binary data IgnoreHWSerNumxxxxxxxxx with value of 1 place the vid and pid on that x.
