Scripted logic could help a bit, but I don't believe there is anything in regular Sage 100 to truly block OVERDIST transactions for FIFO.

Any thoughts on how you would do this with scripted logic?

Script a lookup to IM_ItemWarehouse to check the QoH during line entry in the various business objects.  The big problem with that is accounting for unposted transaction quantities (e.g. two invoices for 10, when you only have 10 on hand...).  Sage has fields for committed quantities, but you also have to consider, if editing a transaction, that transaction's quantity may be in the committed number.  You really have to know scripting (and the way Sage business objects work) to do it properly.

You can use the Lot Costing method. Every time you receive in inventory, give it a lot code. Users will need to distribute from lots when they ship. They can't go negative with this method. It's more work but will ensure you don't get negatives.

For clients who frequently go negative in inventory (due to BAU data entry practices... delays in getting receipts entered, or production posted...) we often recommend Standard valuation.  However, for unit costs that fluctuate, Standard isn't always a great fit. 

Lot and Serial valuations work in those scenarios, but with the extra data entry step.

For FIFO, we suggest emailed alerts (using 3rd party reporting software) of negative (OVERDIST) inventory, but that is only possible after posting.  Prevention is... impractical.

(Speaking of OVERDIST tiers, please vote: Idea).