I'm an OW Emerald and I've seen this problem too, but only on a segment that was full fare Y. It appeared that the manage-my-booking software rule for non-status Y trumped the rule for OW Emeralds on the BA rezsystem (at least in Oct.2011). So...
I paid $75 for an exit row seat assignment, then emailed BA CS about the problem. The next day a rep called to say "Sorry!" and to refund my $75.