We are at 4.2.2 now. Is this already supported, and how do we implement it?
Our plan here is to support lookup on a buffered store but if there is no match we will at a minimum issue a diagnostic message or perhaps throw an error back. The reason being that when a record is not in the page cache, that does not mean it is not "in the store" ... not in the 10M item sense. It means we don't know the answer to the membership question. If the record were being displayed somehow, then we would surely find it and all would be well.
When, when, when?
Many things get exciting during edit. If any set operations are in effect (sorting, grouping or filtering) an edit can completely invalidate the cache. Editing the right field could cause the first record to become the last.
Because the page cache has never supported saving/syncing, this was always a hack in previous versions to get working. We will not be enforcing any checks here to prevent you from trying to edit a buffered store's content, but this is not supported.... still. :)