you know i think the labkey editorgridpanel has a columnModelCustomize event (or something similar). i believe the way you are supposed to manipulate column in that grid is to listen for that event (which is async since the store must load data from the server) and then modify the columns array it returns. that would allow you to all the column, but you may run into a problem downstream because that column wouldnt actually exist in the store. if this does work though it's easier and requires knowing less about the guts of ext.

hooking into the store as i mentioned above is a step upstream of the EditorGridPanel's columnModelCustomize event. it's more work to get it working (although not that much work), but might be more robust.