Actually, I just figured out that with Amaze it is possible to edit the files in place 
So the easiest thing would be to point Amaze to /system/etc/permissions, mark the platform.xml, click “open with”, and choose the Amaze text editor. Then you can edit the file easily there!
In case you have lost the file, you can also point Amaze to /system/etc/permissions and create a new platform.xml file (You can copy the content from the platform.xml you have edited).
Haven’t figured that earlier, but I guess that it might be the simplest solution. Just make sure that you have granted superuser rights to Amaze.