Why not just show up at BRU for the original BRU-FRA flight and let the staff deal with it? It would be apparent to the check-in agents that you would not be able to make it to the FRA-HKG flight and that rebooking would be required. It may be that BRU staff would let you travel to FRA for LH to deal with it there; or if you insisted that they rebooked you from BRU.