I do not want to annoy everyone with the full logic behind this suggested feature but sometimes the time in power zone does not give a fully complete picture of effort dedicated to each exertion level.
A histogram based in kilojoules spent can be much more enlightening specially in narrow zones like SS.
The calculation can be simplified using just the midpoint of the zone in W, multiplied by seconds spend and divided by 1000.
However, it can be much more precise if it takes into account a full watt by watt time distribution aggregated by zone afterwards.
Another variation could be accruing the exertion per zone using cumulative load in zone (TSS) as the measured variable.