I'm not 100% sure and I am sure that someone from MF can clarify it but I am led to believe that once the 900 hour limit is achieved MF crew can be 'reallocated' to other tasks within the terminal that don't involve accrued flying hours.
I think thats a BASSA fantasy.
The course to do anything useful in the terminal (bookings/check in etc) is longer than the course to become cabin crew (6 weeks min). The unskilled terminal work (queue combing, hotel booking) is only really required during mass disruption which is when they get the waterside volunteers in. This is also the time BA need all the crew they can get their hands on to fly.
I guess BA might want crew stuck in CRC during disruption to do some of the above unskilled tasks, but that doesnt seem unreasonable. The idea that BA are going to send CC on a 6 week course because they hit 900 hrs in 10 months seems a little expensive! The whole point of mixed fleet was to stop crew hitting 900 hrs too quickly so it will even out over the year once enough crew are on line.