What do you secure the bag to?

The bag is essentially stuck to my skin with special medical glue... I don't think it'd be strong enough to support scales, even small ones.

 

Unless I'm misreading the Adafruit website, serial out doesn't feed through the USB on the Trinket. It mentions the option of using an FTDI for serial output.

Good catch. I just discover ended that too. They mention two options. FTDI USB to Serial cable or something called FTDI Friend. This would allow debugging the flex sensor.

 

The bag is essentially stuck to my skin with special medical glue... I don't think it'd be strong enough to support scales, even small ones.

Weighing could still be an option - a strap around the not glued face of the bag would be pulled tight as weight bulges thebag.

But you'd need a much more sensitive scale. Probably not going to be a luggage scale, but I think there are various nefarious uses for more sensitive scales with a hook on the bottom.