"However, I would love to see compatibility with third-party levels, such as this one."
You make the level.. server owners add them, boom. Problem with this level is the original creator hasn't updated to new release/engine. And we don't wanna go rip off his work.
If a third party level is good enough, I'm sure the team would add a full complete version of it to the next release, or give it support/publicize it on forum/frontpage/moddb.