If you're disadvantaged by being in the US at the time that OLCI opens for the intra-European flight, then surely the answer is to make sure that you fly across the Atlantic on BA as well on the same ticket? Then you would be able to check-in for the intra-European flight at the same time as checking-in for the trans-Atlantic, even though it's 30+ hours before the departure of the intra-European flight.

So - apart from the principle of seat selection - if the timing of OLCI "sucks" for this reason, surely that's only because you're choosing to put yourself in a position in which you're really not very valuable to BA, namely by flying a different airline trans-Atlantic?