It's an interesting question, I had always interpreted it as a 'blanket limitation' not just applicable pre dispatch. Nothing in the FCOM wording points towards it being a pre dispatch limitation only, so I would suggest my interpretation is correct. I'm sure someone could think of a scenario where it would be justified, but I wouldn't consider the instance you gave as being one of those. For me it would have to be safety related (maybe a scenario involving a leak of some kind?). Also you would need to burn the best part of 2 tonnes each side to ensure that CSFF would not occur, although I'm sure that may differ depending on the variant (in our 738 operation we plan to land a max of 4.0T in an attempt to eliminate the threat of CSFF. Our wings typically hold 7.8T).