This is what I finally did, maybe is usefull for someone.
I introduced two fields,
in which key is subject, and value is a list of elements (Teachers) that may give that subject.
private ComboBox<TeacherComboCustom> teachersCombo
I also introduced a Renderer to the Teacher Column, where it looks into the map if that subject has already teachers. If it has not, calls an RPC method, and on the onSucess method for that call, introduces new values into mapSubjectIdListOfTeachers.
The editor for that column, on the preProcess, cleans teachersCombo (removes all from the store), then goes to mapSubjectIdListOfTeachers to get elements to add to the store.
Hope it helps someone. Not very proud of what have I done, but works fine!