You could use extended texts as an aide memoir.
Obviously not as neat as automating requirements but can work as a suitable solution. I used this method very effectively in a purchasing department where we had complex mixes, of min x qty per SKU, min y qty per SKU group and a potential minimum order spend for the whole order.
Writing the logic for MRP to do this would have been an expensive nightmare whereas an extended text just lists the requirement as a line of text for the user when adding that item.
Note, it is item rather than vendor specific. If you have multiple vendors for the same product, this will have limitations.
Recurring purchase lines may also give you a partial solution by showing what other items are from that supplier within the PO itself. Note this would be a workaround and could be quite fudgy but may be perfectly acceptable or at least an improvement.
