The issue is that we can't calculate how many will be made with buymax if it is not a "permitted" creation method, since it will not be part of the concoction calculation in that case.
We could mark the mixing methods as permitted - but then the concoctions will show up on the Createable tab of the Item Manager, if you have the ingredients.
I suppose that NPCPurchaseRequest.parseShopRowResponse could handle this: if buymax=on and item.getQuantityPossible() is 0, it could look at the response text and find the "You acquire" line to figure out how many items were acquired:
Code:
<td valign=center class=effect>You acquire <b>4 giant erasers</b></td>
Perhaps you would enjoy coding and testing that, since I won't be in KOLHS for months, if ever.