I've been able to reserve economy seats on SQ by calling them directly, but neither they nor LH would reserve bulkhead or exit row seats for me in advance (I'm UA *G). LH's excuse was that they had to see me in person to decide if I was fit enough to assist with emergency procedures if needed (I said they could assign me the seat and then take it away at check-in if I showed up in a wheelchair, but they didn't go for that), while SQ said they wouldn't assign bulkhead seats until they saw how many people showed up with infants and needed bassinets. In the end I got the seats I wanted on both airlines when I checked in, but it would have been nicer to have them locked in in advance.