It would be *far* easier to modify the stored procedure that drives the grid. And when I say modify, I mean take a copy, rename it, and change the system setting to point to your custom stored proc!
In the case of the Warehouse Summary tab/grid, I see in System Settings->System Configuration->Inventory that there is a setting called "WarehouseSummaryStoredProcedureName", and it's value is "usp_JIWA_Warehouse_SOH_Summary". So, follow these steps:
1. Make a copy of "usp_JIWA_Warehouse_SOH_Summary", and name the copy "usp_LONICERA_Warehouse_SOH_Summary". Don't forget to run "grant_all_user_tables" to give the correct permissions to Jiwa on your new stored proc.
2. Change the setting System Settings->System Configuration->Inventory->WarehouseSummaryStoredProcedureName so that it's value is "usp_LONICERA_Warehouse_SOH_Summary".
3. Now go back to the Inventory form (close and re-open it if it's already open), and ensure that the Warehouse Summary tab/grid still works.
Now it is time to make your actual code changes to the "usp_LONICERA_Warehouse_SOH_Summary" stored procedure. Looking at the stored procedure code, I see that it is not actually returning XML for grid construction/population - it's a bit of an ugly duckling in this regard. Nonetheless, I find that if I simply add a column to the "#tmpWarehouseSOH" temp table, populate the column, then spit it out as part of the final SELECT query of the stored procedure, my column appears on the Warehouse Summary grid! Too easy!!