playing a slave would be interesting, since the effects of torture on one's personality is quite profound, however, integrating such a character effectively into this RP would be intensely difficult. He cant be a side story on his own as a character enslaved throughout the RP because the point of RPs is that it is a communal creative effort. However, he would not want to actively fight the slavers once he has been freed, nor would he have any interest in joining any of the captains aside from getting a job. Even then, there's no reason why the captains would hire him. They have their crews, there's no emotional investment in him and what with malnourishment and extreme mental instability, he's not fit to be a sailor, particularly when considering the horrors he has seen on a ship.
So it's interesting, but just wouldnt work.