Well it seems SQ are restricting seats to bmi.

I booked two seats for IST-SIN despite ANA showing only one.

After which, another one popped into the I class inventory.

The seats were showing on ANA, Air Canada and I was even able to hold the seats with US Airways. I thought that by doing that and cancelling them shortly afterwards, they might be open to everyone. Seats did reappear back in ANA but alas nothing for BD.