Hi Crispin,
I can't comment on the technical aspects of that customisation, because that is just not my role!
I don't think I have ever been asked to implement FIFO on items which AX does not really know which inventory was received first. And, let's be clear, anything you do without using the Batch dimension and Batch dates is a best guess. For example, if at any time you put multiple receipts in to the same location, then it becomes impossible for AX to distinguish between the inventory received on different receipts just as it is impossible for the picker to determine; you have no way of knowing that you actually received the items in the order in which they expire or were manufactured.
When picking items which are not Batch controlled, there are often rules around which location to use first, but FIFO is not one that I have used, and if it were crucial, I would suggest using the Batch dimension.