I was thinking about this last night too. This would also help if you’re doing rides with a group where the majority only have power meters, since they can easily hold steady power cresting a hill or downhill, which makes drafting with a Kickr a bit more challenging.
As you said, it would also allow Kickr users to do FTP tests or other programs (TrainerRoad) on Zwift where you require steady output (which the constantly changing resistance makes difficult.) I’d never want to give up the dynamic resistance feature but having a toggle for those certain occasions would be awesome.