It would be very useful if all collection type APIs supported filtering. This would be useful because in many cases the user may want to type into a filter box to reduce the result set. Not only in displays of collections but also this would enable any of the collection type APIs to be used for backing form controls where type ahead is supported.
I think this would be a valuable addition as standard for all APIs. By default I would expect a basic "term" parameter to supported - what that term maps to would be down to the API.