A workaround we’re currently using that might work for you is to set up all tasks with a relative (estimate) date and link them with dependencies. You can then later move them as a cluster to the correct date once it is known, using Dependency Management Options
This might be a bit convoluted and confusing since you’re not only setting a relative distance, but also claim that these tasks are interdependent. Then again, I think it’s the closest you can come to a solution with current Asana functionality.