Interesting idea. There is an option for the Sunday ride in most plans. It is only housed in the Weekly Tips, where you can choose to do a longer Endurance workout vs the shorter Sweet Spot one that is in the plan.
But it is text only and must be added by the user manually. I think some more integrated solution would be great for that existing substitution option, and could be extended to your suggestion.
The issue I see is time to build and maintain that additional set of workouts. Not insurmountable, but it would add to the work list.