I thought this happened over 4 years ago, although if true, would support the hypothesis that it might happen again...
Well it couldn't happen any more recently could it... there haven't been any DEPs...!
Post 9/11 BA, to their credit, honoured all contract offers, even if unsigned, since people had handed in resignations to other airlines, who post 9/11 were very glad to see volunteers "go". BA gave some menial jobs, farmed some out to CitiExpress etc., but basically honoured them.
Some of these people had 777/744 on their contracts. They are still flying Airbus'. Some are bitter, some not...
As for "how extensively" I do not know... We have had new guys shunted to and from the 757/767 and Airbus against their will, or posted to the regions (MAN, BHX) and as soon as they have signed for a house up there, shunted back to LHR.
It is
not ungentlemanly - it is in the contract / T&Cs. As a wild guess, anyone offered a 777 course for say Feb, is, say, 75% likely to start on the 777. All I am saying is keep your eyes open, and what really matters, if BA is for you, is your start date. Delaying for even a few weeks to get the fleet you want could cost you hundreds of ŁK and leave and command etc. over the coming decades...