Can this be done for all models? My impression was that this is no longer possible in many cases.

Just like Chromeboxes, some models require the removal of a screw or a jumper while others need a special USB cable (cheap and easy to diy). I don't know about other models preventing the unlock, but if they do exist I would avoid them at any cost.

You can find here if your model is supported: https://mrchromebox.tech/#devices

You do not need the special cable.

Just disconnect the battery. Connect power adapter. Power ON. Follow https://mrchromebox.tech