Passing data back to a child record from parent table in order to filter the list of records available to relate to parent

Okay, hopefully I can explain what I need here. We build records in a table called defects. These are reported problems that need to be fixed by work orders. The defect records sit in their own application. Work orders are developed in the other application and have the work order table set up as a parent to the defects table. When we start a new work order, we select one or more defects to fix with the new work order. I need to flag each defect that becomes assigned to a work order so it drops off the list for the next selection of a defect to be assigned. I've tried creating a lookup field (checkbox that goes to true when a defect is selected for the work order) from work orders in the defects relationship, but cannot get the field to populate in the newly attached defect record so it will be filtered out from the to be assigned list.