Is it permissible for wives to visit their imprisoned husbands while wearing immodest clothing (such as strapless tops) with the aim of comforting them, given that the husband is the only one who sees them?

It is permissible for a woman to adorn herself for her husband and reveal her allure to him at any time. There is no harm in this within a prison, provided two conditions are met: First, no non-mahram person sees her, whether directly or through surveillance. Second, it does not have a negative impact on the husband, such as arousing his desires and emotions to the point where he compromises his religion and longs to return to his family and children. Allah has warned that family and children can be a trial (fitnah) that prevents a man from fulfilling his religious obligations. Allah Almighty said: (Say: If it be that your fathers, your sons, your brothers, your mates, or your kindred; the wealth that ye have gained; the commerce in which ye fear a decline: or the dwellings in which ye delight - are dearer to you than Allah, or His Messenger, or the striving in His cause;- then wait until Allah brings about His Decision: and Allah guides not the rebellious.) [At-Tawbah/24]. And He said: (O ye who believe! Truly, among your wives and your children are (some that are) enemies to yourselves: so beware of them! But if ye forgive and overlook, and cover up (their faults), verily Allah is Oft-Forgiving, Most Merciful.) [At-Taghabun/14].