You could place a button next to the field that copies the text. You could use OnObjectKeystroke to perform a script each time a key is pressed while the cursor is in that field. This script could use Get ( TriggerModifyKeys ) to detect that a modifier key such as the control key is down so if Get ( TriggerKeystroke ) returns "c" and Get ( TriggerModifyKeys ) returns 4, ctrl C has been pressed and you can use the copy script step to copy this data to the clipboard.
And you might find it easier to not use Kiosk mode. You can use custom menus (FileMaker Advanced) and manage security to lock a FileMaker solution down pretty tight without going all the way and using Kiosk mode.