I found someone who made his teensy in to a usb mass storage device, he did use a 3.3V regulator.
it does mentione that is's slow but he got the sd-card mounted and from there you could use irongeeks code to look for it and run some code... I don't know if it would be possible to use the HID functionality while doing this tought.
The source code is on the page, but i'm not skilled enough to try this myself