I found out how to do it. In order to sort by key, you just need to use __key__
as property. For filtering there is a convenience method called filterKey, but its equivalent for sorting (orderKey), as far as I can see, does not exist.
ofy().load().type(ActionIndex.class).order("-__key__").limit(10).keys().list();
--- Edit ----
Based on the comments below, it appears the use of a Date property is compulsory.