Toby, consider that even in the event of an EFATO, you push to level off and accelerate in level flight. They had 2 engines, thus they would be both higher and acceleration would be correspondingly quicker. I expect that there would be more than enough margin climb performance.
Is it a requirement to disconnect if the aircraft does not maintain VLS? I often see pilots 'adding a few knots onto Vapp' because they are worried about reducing below Vls - but Vls is the lowest selectable speed, not the stall speed. The aircraft might lag a bit when heavy but give it a chance...a few knots below Vls for a few seconds while it adjusts the pitch attitude isn't going to kill you.
However, as ALK said, the key is/was (with 20/20 hindsight and knowledge of the FCOM), wait until well above S speed before retracting in the case of turbulence and/or very heavy weights.