I really don't see the issue. The centre tank is close enough to the CoP that trim requirements will be well within the available range, after all you can get airborne with 7.8t and deal with the CoG change in flight, and the CoG change here will be comparable. The start point of the CoG may start and finish a little futher forward with this addition 5t in the centre, but it's highly unlikely to have any adverse effect once in flight. Ultimately, there is little you can do should it actually happen, so any calculated trim and MAC is pointless. Trim what you need. Stability will be greater with a fwd CoG, and handling may be affected by needing a bigger control input for the flare, so if the aircraft needs a lot of NU trim before descent, you could move some passengers aft to compensate, but unless you really want a pointlessly academic hypothetical debate, I would suggest it's enough to just say land asap at a field that will allow a gentle landing at high weight.