Vantage - Updating UD field after Shipping

I'm working on a OTIF reporting for our Ops team and we want to assign OTIF exceptions to shipments (for example, the customer haulier didn't collect when arranged). 


This all works fine on the ShipHead information, however there is a  possibility that not all the ShipDtl should be counted in the same way as we may have been behind on shipping a certain line anyway.  What I would like to do is run from the ShipHead with a BPM duplicating the info ShipDtl (as this is quickest for the user and hopefully will cover most problems) but I want to be able to update the individual lines on ShipDtl if necessary.  The brick wall I am hitting is that as soon as the PackNum is Shipped (Ready To Invoice) then I can't change any fields, not even UD ones. 


I have also looked at using Invoices with FKV but no joy.  Any suggestions?  I believe I could use an updateable dashboard if we were on E9/10 but unfortunately that's not an option.  Thank you in advance.