It might be enough to just remove these checks https://github.com/10gen/mongo/blob/3c4288c4b68cbb881678e4ccefca7a308596e037/src/mongo/db/commands/query_cmd/distinct.cpp#L470-L488 and add some tests. If it turns out to require more work than that, we might not want to do this as part of the shard filtering project.