Could i use the Teensy 3.1 and have the usb plug into a usb slave device that requires a host and open a serial connection to that slave device with the regular Serial.write()?
I would guess that the Teensy is not capable of acting as a USB host, like a pc is. But to be sure I would ask the developer:
https://www.pjrc.com/about/contact.html
[EDIT] Nevermind, I see you have already posted in the forum there.