I don't know if this helps you (or anyone), but I can confirm that the serial component in AI2 does work with an arduino device using an OTG cable. I am able to initialize and open a connection and then send information to the arduino to blink LEDs (HIGH/LOW), etc.
In terms of information transmitted, the arduino (and probably other devices) can convert ascii data into hex/binary/etc upon receiving or before transmission. Not sure about your gamepad if you can't program it.
Re: USB cable occupied, my workflow for this stuff is:
- From app inventor, build app to QR code.
- Copy the link (it's always the same except for four characters) and download on phone browser and install.
- To update, delete app and repeat. USB cable remains attached to serial device.
See this thread for a working .aia:
https://community.appinventor.mit.edu/t/usb-permission-popup-every-time-serial-component/12596
I can provide you with a sample arduino sketch if it interests you, but it seems like you're working with other hardware.
I have not worked with receiving data from serial device yet, only transmitting.