While debugging the UpdateCommand event were you able to verify that e.Item is in fact a GridEditFormItem? If not, I'm guessing that using InPlace for the edit mode causes e.Item to be a GridDataItem. If that is the case, then you should be able to get your column's value by casting e.Item as a GridDataItem and referencing the hidden column that way.
I hope this helps!
source, GridCommandEventArgs e)
//Breakpoint here to determine what e.Item really is
GridDataItem Data = (GridDataItem)e.Item;
Hashtable ht =