Interesting suggestion, speed in turns is one of the reasons (not the only one, but one of) why the speeds in Zwift are unrealistic.
The suggestion to set a maximum speed and a braking zone to each turn depending on the turn characteristic & complexity should work fine. In other terms, if you are going 30k over the turn speed set limit, the braking zone will be longer, then if you were only doing 15k over the turn speed limit. (However it should not be linear, the more speed, the braking should be a bit more aggressive)
On a smart trainer, the load could be set to 0 on the «braking zone» prior to the turn, then on the exit the load could be the one required to bring you back to the speed before the «braking phase».
On a non-smart trainer, it could work on a similar way (Watts done in the braking zone would be overrided to 0, and speed overrided to the required deceleration speed). Then you would need to do the watts to get you to your former speed at the exit of the turn.
Soon Zwifters will develop the habit to stop pedaling while cornering, because continuing would be just a waste of energy.
To get even more realistic, in the Braking zone, the screen could be offseted toward the turn. In other terms, instead of looking straight ahead in the braking zone, you would look on your right before a right turn or left for a left turn.