I donít think that being part time will make it any more difficult. I am a FT student who is an extern, and a few of the other externs are part time students. I think the main thing is that you have to be able to take some time off of your present job. If your job is OK with that then the court should be as well. Also, my externship is PT, and I would think that most part time judicial externships want you to work during the day, so you would have to be able to take about 16-20 hours off a week during the day from your regular job.
Dunno about smaller firms.