True, true. May be that some of the merchants caravans would be open enough or have a somewhat mixed population, helping to confuse people who would want to persecute them, with the Christians among the group being sufficient to convince locals that the group is just foreign. A bit of mercy here and there from the Jews to those who are abanonded or lost in the woods and I imagine that some of the children and adults would wish to convert to Judaism.
Do you have some sources on this? I am not doubting it (as thralls, serfs, peasants, slaves, and unfree labor was around at different times throughout Europe for ages) but I am wondering which groups and time periods in specific did this and if the Magyars and Vlach responsible for this did the same to their own people or to prisoners of war.