Hi John,
As you know, in the product master, the lot size maintenance is time-independent, so that obviously can not be used for your scenario. As per my understanding, to meet this using standard functionality, you can maintain multiple product specific t-lanes (for STRs) and PDSs (planned orders) with required lot size ranges and validity dates (for short or long term horizon). But, again since the validity of t-lanes and PDSs will not be automatically rolling forward so the updating the validities will be a challenge.
The other option could be to enhance the heuristic functionality at lot size selection step while creating order.
Regards,
Umesh