I have never heard of a formal ortho externship.
I think it would be better to go and visit the ortho programs you're interested in for a day or two. The reason OMS has externships is due to the huge variety in programs and what each program has to offer. Each program has a unique emphasis, some cancer, some trauma, some craniofacial, etc... At most externships it's extremely hands on as you take trauma call, work in the outpatient clinic, help out in the OR, round on patients and attend case conferences. You get a good idea of the hours you'll be putting in, the residents you'll be with for 4-6 years and the variety of facilities and hospitals you'll be working in. An ortho externship would be somewhat blah since you would probably just watch the residents work all day and maybe get to place some brackets, change bands, do some tracings and sit in on some lectures, which would make 1-4 weeks very long.
Just my opinion.
tjb