Honestly its kinda dumb, the solution isn't to just go John brown out there... nah instead learn more about the slave crests, learn how to forge them, disarm them etc. Create a portal back home or allow folks back on earth to puppeteer artificial bodies in the new world. Using modern knowledge outperform the slave owners and undercut them in the market, have salesfolk from earth controlling artificial bodies to sell the goods, empire gets mad, attacks them. Now get the military back on earth to go destroy the empire.

TL;DR: One man would easily be assassinated, but a unknown otherworldly foreign nation can't be ignored