How can I add a column to multiple tables at once in MySQL Workbench?
You can use this Python Script in the Scripting Shell of MySQL Workbench. It is an improved version of the script found here: http://mysqlworkbench.org/2012/06/mysql-workbench-script-for-adding-columns-to-all-tables-in-a-model/

# define your columns and types here
columns_types = {"created_at" : "DATETIME", "updated_at" : "DATETIME"}

# tables you want to skip
skip_tables = ["main_defs", "main_menu_items", "delayed_jobs", "delayed_job_workers", "delayed_job_logs"]



# get a reference to the schema in the model. This will get the 1st schema in it.
schema = grt.root.wb.doc.physicalModels[0].catalog.schemata[0]

# iterate through all tables
for table in schema.tables:

    # skip the current table if it is in skip_tables
    if table.name in skip_tables:
        continue

    # iterate through all columns to be added
    for column_name, type in columns_types.items():

        # skip this column_name if there is already a column with this name
        column_names = [x.name for x in table.columns]
        if column_name in column_names:
            continue

        # create a new column object and set its name
        column = grt.classes.db_mysql_Column()
        column.name = column_name
        # add it to the table
        table.addColumn(column)
        # set the datatype of the column
        column.setParseType(type, None)