I'm sure there's plenty of use cases for this... but I personally can't think of anything I can do online that would make the authorities more dangerous to me than dropping of an antenna laden, home-made looking spy-box at my local coffee joint.

Posted recently https://news.ycombinator.com/item?id=9811218

I'm not sure how this works, because 900 MHz is a ham band, the FCC forbids encrypted communication over this frequency

It is, but 900MHz is also used as the cordless phone band, and those are often encrypted.

Now you just need those face-morphing pills that Tom Cruise takes in Minority Report to avoid being caught on video tape sneaking around your local public library with some sort of transmogrifier-looking device.

Not quite the face-morphing pills you're looking for, but something to the same effect http://cvdazzle.com/

You could get one of those latex masks that Tom Cruise uses in Mission Impossible.

Okay, not quite as good, but good enough to disguise yourself and not arouse suspicion if you keep your interaction with others to a minimum. (Here's an example from Penn and Teller's TV show. The effect is likely diminished if you are expecting it though: https://youtu.be/7P3B6R6Eqwc?t=1797)

Masks like these are commercially available from mask-makers like this: http://www.spfxmasks.com/ourmasks.php

I'll stick with my motion-detector triggered thermite laced bitcoin bought servers thank you very much.