Chris;
Your question, and your caution are very well placed. Indeed, one must be very careful with the procedure.
The autothrust system maintains speed well enough that action shouldn't be required. The question arose as to how the pilot might obtain more thrust if in the pilot's judgement such is required; - a momentary selection above the CL detent can be made, but you are absolutely right...it can be over-done so must be used with great caution - a second or two would be all that would be needed, and that is from my experience on the A320 and A330. Even in the last 20ft, there is plenty of residual thrust as the thrust levers are closed, (precisely because of the behaviour of the FADECs as you have described) and the engine response, at this point beginning the flare, is much quicker than has been assumed.
In fact, in researching the question, I couldn't find the procedure in some manuals familiar to me so at least some have decided it isn't necessary for normal ops. Perhaps the risk of inadvertently going further, into TOGA, was deemed greater than the small benefit. In my experience, the autothrust system does an extremely good job.
Last edited by PJ2; 1st Mar 2011 at 21:33.