I can confirm that BA did (can't say if the still do) hold some seats for premium card holders.
When I was gold I tried to book an open jaw (LHR-YVR x LAX-LHR). I couldn't do this on line so I called up the crappy ExClub desk in Bremen.
Before I could even speak I was told that unless I had a request that I couldn't do on-line I'd have to pay a fee. He then agreed that I couldn't do an open jaw myself and I wouldn't be charged.
Just before the call I had been onto the BA site and had determined the seats I wanted in WTP were available on both flights. The agent could see the seats on the Vancouver flight, but couldn't see them on the LAX flight. I checked again and saw they were available to me. Oh, he said, I don't have the same priority as you do. He then said - it won't be more miles if you book two one ways rather than an open jaw, so I'll book you on the YVR flight and you book the LAX flight yourself. And that's what we did.
Guess what, next credit card statement had a charge from BA (separate from the two taxes/fees/charges on the tickets). Turned out that they had charged me the fee. When I called them to remind them that they had agreed not to charge me I was told - You could have booked the YVR flight yourself, you didn't need us to do it. They refused to waive the charge.
One more reason why I don't fly BA.