tubby Linton: just brainstorming here and I know "it's not in the FCOM" but in your scenario, since you still have both engines running, you could if really pushed, use less than equal thrust on the two engines to reduce your fuel imbalance once the cross feed is closed and the gravity feeding is fully established! Very inefficient way of flying but you should be able to retain normal cruise levels. Difficult to see how in real life this scenario would be an operationL issue.
Tin hat on for incoming.............