The Hanafi view as mentioned in Hidayah (arabic, english):
A man may look at the female slave of another, in the same manner as
at his kinswoman (mahram). EVERY part which it is lawful for a man to
look at in his female relation, may likewise be viewed by him in the
female slave of another, whether she be an absolute slave, a Modabbira
a Mokatiba, or an Am-Walid ; for as a slave is necessitated to wear
clothes adapted to servile employments, that she may discharge the
business of her master, and attend upon his guests, her condition
without the house is therefore the same, in relation to stranger, as
that of a free woman without the house, in regard to her kinsman. With
respect to privacy, or travelling with the female slave of another,
many have said that it is allowed, in the same manner as in the case
of a female relation. Some however, declare it improper, as not
being justified by necessity. Mohammed; in the Mabsoot, has said
that the assisting of a female to ascend or , descend from a horse is
approved, provided it be in a case of necessity.
The Shafi view as mentioned in Imam Nawawi's Minhaj Al Talibeen (english) is:
As to a female slave any one may see her body, except between navel
and knees ... The best authors consider a female slave as subject to
the same rule as a free woman, so far as regards the prohibition of
being seen.
Also might be useful to study tafsirs on 33:59, the covering was seen as a status symbol that differentiated the free women from the slaves and the non-muslims (though I once came across some samples of pre-islamic poetry that implied that the ancient Arabs also considered it as a differentiation between the free women and the slaves).
From Tafsir Al Qurtubi (Qurtubi was of the Maliki school):
السادسة: قوله تعالى: { ذٰلِكَ أَدْنَىٰ أَن يُعْرَفْنَ } أي الحرائر،
حتى لا يختلطن بالإماء، فإذا عُرفن لم يقابلن بأدنى من المعارضة مراقبة
لرتبة الحرّية، فتنقطع الأطماع عنهن. وليس المعنى أن تُعرف المرأة حتى
تُعلم من هي. وكان عمر رضي الله عنه إذا رأى أمَة قد تقنعت ضربها
بالدرّة، محافظة على زيّ الحرائر. وقد قيل: إنه يجب الستر والتقنعّ الآن
في حق الجميع من الحرائر والإماء. وهذا كما أن أصحاب رسول الله صلى الله
عليه وسلم منعوا النساء المساجد بعد وفاة رسول الله صلى الله عليه وسلم
مع قوله: " لا تمنعوا إماء الله مساجد الله " حتى قالت عائشة رضي الله
عنها: لو عاش رسول الله صلى الله عليه وسلم إلى وقتنا هذا لمنعهنّ من
الخروج إلى المساجد كما مُنعت نساء بني إسرائيل.
Number 6: God says: "That they will be known" This is more suitable
that the free women are distinctly identified so they are not confused
with the slave women. When they are known, people will show reverence
and respect to them and will not come in their way due to their
status. It is narrated about Umar that when he saw a slave woman in
hijab he punished her, indeed he did that to protect the [sanctity?]
of the clothes of the free women.
One saying is that: Now all women, free and slaves are obligated to
observe the hijab. This is similar to how the Sahabah prevented the
Muslim women from visiting the mosques after the death of the Prophet
(p.b.u.h), whereas the Prophet had said: "Do not prevent the women
servants of Allah from the Mosques"(Bukhari), in so far that
Ayesha said: "If the Prophet was alive in our time he would have also
prevented the women from visiting the mosques, just like how the women
of the Jews (Israelites) do not visit the mosques (synagogues)".
