Nice necro thread!
It depends completely on the culture of the school. My instructor is a black belt under Roberto "Maguila" Marques, and Maguila has told us during seminars that if we're ever in the Washington, DC, area that we can drop in and train for a week free of charge.
If you belong to a larger network of schools, such as Gracie Barra or Renzo's schools, they might be more accepting of something like that. But I don't think any serious or committed instructor just wants you bouncing between schools on a regular basis.