Can I install a humidifier on a horizontal duct?
That's plenty of space to install a humidifier, provided it's air intake scoop is designed to operate or can be installed horizontally. Freezing of the supply line or its drain is a concern; steam humidifiers are recommended for unconditioned spaces. Drain it inside, install a catch pan and a shut-off switch, like you have for your AC coil. Sharing that installed pan's drain and safety switch will not help; it only shuts down the AC condenser, not the entire furnace.