I've once read an Erlang/OTP developer about patches from external contributors. He wrote that virtually all patches they receive have to be rewritten basically from scratch. The patches break existing stuff (automatic tests are useful to catch this), are using an other coding style, don't contain documentation comments, etc. In the end it takes a lot of time and effort to manage these patches, nearly that much time it would take to develop the code internally from scratch.
I do think that the situation with the LWN engine would be very similar. I suppose the editors don't have much time to work on the engine, so they wouldn't have much time handling those external patches. So these patches would just fill a mailbox somewhere...