It may be tied in that the cabin crew's ipads now seem to show non-BA status passengers as either Blue or non-member - I'm not sure which. When they click through to a lower layer they see the actual OW status. That would seem closer to the lounge access database than anything else.

But in terms of seat selection, the only booking I've had since I put my MH Platinum number in resulted in me getting all the usual free seat selection, including exit rows on SH, right from the TA booking stage (and I don't have a free seat selection deal with the TA), so that part of BA's IT is working.

You're not going to be penalised for having a different airline's status. That would cause a lot of angst amongst AA EXP if nothing else!