are there any 'clues' to identify this as 'indirect discourse', or is it likely or common for κηρύσσομεν [the verb in the first clause] to be implied in constructions like this?
With ἑαυτοὺς δὲ δούλους you've got two accusatives, which you often have in indirect discourse. οἶδά σε δοῦλον εἶναι. "I know you to be a slave, I know that you are a slave." On the other hand, you certainly could just take ἑαυτοὺς as the object of κηρύσσομεν with δούλους then being in apposition to ἑαυτοὺς. Either way the meaning comes out the same.