Hi Marc
I took some time to think about your question, as it is an interesting business requirement.
However, I could not find any reasonable solution on the standard system.
I don't think that it is possible to achieve this kind of behavior with any of the standard lot sizing procedures.
In fact, it is even difficult to create a solution through MRP using a BAdI.
Since it is a very specific requirement, maybe it would be better to crete a separate solution, using BAPI_PR_CREATE, to read your requirement and create the purchase requisitions, to be executed before MRP.
I cannot think about any standar solution to resolve this issue.
BR
Caetano