I admit it is a difficult question and one's natural sympathy would have to be with the sex slave.
If the actual prohibition is based upon that all sexual intercourse should be open to procreation it is difficult to see how even a sex slave can justify the use of a condom.
The converse argument is that its use as a prophylactic in an act not consented to by the sex slave is not primarily intended as contraceptive. The suspicion is that it could be both of equal importance.
Despite this, such is human nature that exceptions that are made for hard cases are quickly applied to 'soft cases'. Catechesis, the teaching of the Faith wherefrom most 'ordinary' Catholics learn their Faith and Morals nowadays comes via the secular media and alas, via headlines and soundbites, most Catholics do not read Catholic publications and the classroom and the pulpit have long since ceased to be where Catholics learn and study their Faith.
I would leave this one to God.