As soon as you save the migx-item the first time you should get a field with name 'date' and the unix-timestamp of the time, when you opened the formtabs-window. This value should not be overriden the next time you are editing this item.
I got this in my test:
[{"MIGX_id":"2","title":"test2","image":"","description":"","resource_id":"","hiddendate":"1359462335","richtext":""},{"MIGX_id":"1","title":"3","image":"","description":"","resource_id":"2","hiddendate":"1359462345","richtext":""},{"MIGX_id":"3","title":"test","image":"","description":"","resource_id":"","hiddendate":"1359462357","richtext":""}]
now getImageList can sort by this field
&sort=`[{"sortby":"hiddendate","sortdir":"DESC"}]`