Hi Luke,
With my limted knowledge of ABAP i was trying to understand the flow of HU number generation (V51S_CREATE_HU_HEADER) and found that technically system creates VENUM first and then EXIDV
EXIDV generation
and after generation of this there is a customer function available to update the value in VEKP at line number 364 at function module V51S_CREATE_HU_HEADER
so technically it looks possible however still the query is how the trigger will go to run the function module to create VENUM also i will suggest you to consult with your development team on this and one more thing to remember here that after all this there is a update program runs to update the status of HU so if you are making any change make sure the further status and other parameter are not getting change otherwise u will face inconsistency and probably will get some standard SAP error