Using proportions and maintaining exceptions with?

We are planning on using proportions generated based on past history to be the basis for disaggregation and aggregation for our key figures. My question is if we have exception to this where for certain periods of time we need to manually maintain proportions where is the best place to maintain this?

For example product A can be produced in plant M or N. For a certain customer ship to US it is made in plant M. But for capacity reasons during the months of July through September we would like the product to be made in plant N. What is the best way to maintain these types of conditional rules? Is it using a combination of proportions and Product/Location Split or do we need to use proportions and some manual maintenance of Macros?

Regards

Jeff