My experience has always been that although you can check in for your onward LH flight you cannot download your BP until there has been a manual passport check. Whenever flying to the US from NCL via LHR this would usually be done in the lounge there.

I have found that some carriers won't let you check in at all until they have done the check. For example, flying HKG - LAX on CX on Saturday I couldn't check in online and had to do so at the desk at the airport, and equally when flying out to HKG then I couldn't check in for my LAS - LAX sector with AA although CX was perfectly happy to let me check in and get my BP for their sector.

In any event, it is good news that the whole process is a lot more explicit and obvious during OLCI