When using the demand planning/MRP, is there a way to define a minimum order quantity?

No, this is a gap.

Is there a workaround solution? Maybe a script that overrides the quantity in the Item Supply Plan (for Time-Phased) or Planned Order (for MRP) if it's below the suggested MOQ in the Item record?

It's a big gap. It uses Reorder Multiple as the Minimum order quantity. We worked around this by creating a Minimum Order Quantity field on the Inventory Item form and then sourcing that to the Purchase Order Line so the buyers can at least have it as a reference when finalizing their PO's.

Gotcha. Thanks. We'll try to impose it on the Item Supply Plan and Planned Order record.

I have added a field to the item master previously then created a saved search of transaction lines versus that item field. Set it up as a portlet or alert for PO Lines to Review

@Rich Keller thank you. That's a good detective measure. I'll see if that works for our team.