Here is my solution. Add the code below to the bottom of your settings file.
# Process --set command line option
import sys
# This module can be imported several times,
# check if the option has been retrieved already.
if not hasattr(sys, 'arg_set'):
    # Search for the option.
    args = filter(lambda arg: arg[:6] == '--set=', sys.argv[1:])
    if len(args) > 0:
        expr = args[0][6:]
        # Remove the option from argument list, because the actual command
        # knows nothing about it.
        sys.argv.remove(args[0])
    else:
        # --set is not provided.
        expr = ''
    # Save option value for future use.
    sys.arg_set = expr
# Execute the option value.
exec sys.arg_set
Then just pass any code to any management command:
./manage.py runserver --set="DEBUG=True ; TEMPLATE_DEBUG=True"