My guess is the printer port is memory-mapped in the cartridge space, something simple. DBT used CRU bank switching in its cartridges but, with the exception of later models, the circuitry does not look complex enough for that.
Do you imagine that outputting to the printer was writing to that address space and that data was the datastream for the printer? Or do you think somehow reading a certain section of data was what caused the data stream -- where a read access is handled as a signal to output data. Doesn't looks complex enough for me. I guess I'm just coming from the notion that you can't write to the cartridge port. But then, I guess there's MiniMemory, though, right?