Hello
There is no such functionality to make the dependent requirements available for planning only at a specific point in time. However, you can implement your own logic to make dependent requirements available for MRP or not using BAdI MD_CHANGE_MRP_DATA.
See the blog Making an element not relevant to MRP or MD04 using BAdI MD_CHANGE_MRP_DATA with an example of this BAdI.
BR
Caetano