The effect of taking shaft power from a windmilling engine is to decrease rotational speed and airflow. The airflow reaches a minimum at locked-rotor conditions. Internal drag will therefore change (see references below).
The spillage drag is also likely to increase due to the reduction in airflow through the engine. Spillage drag will be a maximum at locked-rotor conditions.
See also
http://aerade.cranfield.ac.uk/ara/19...-rm-e51k15.pdf
and
http://ntrs.nasa.gov/archive/nasa/ca...1993094390.pdf