I am not aware of any basis as well. Probably they are against it but they do not put any such condition when the woman/young girls applies to the Darul ul Uloom. Probably the onus here is on the women/young girls to make sure that they take all precautions when traveling to the Darul ul Uloom, and the latter is no way responsible if ever they dont. Although they do not warn anything about it. Is it possible that a certain leeway is adopted in this situation? for example, if there is no Darul ul Uloom in a country and the next best alternative is one in a country which is more than 50 miles away. Then the women/young girls can travel without mahrams provided they are in a big group of other women/young girls?