While it's not USB, the "Planet Bike Rack Blinky" seems like it's otherwise what you're looking for -
Rack Blinky 5
The tail light bracket says "25mm between each of the top horizontal holes (center to center)", so since there's 3 holes, I assume it means 50mm between the outer 2 holes -
Taillight Rack Bracket
There's also other possible mounting hardware, though I doubt you would need it -
I am a fan of Niteflux lights. They can be mounted in most places with cable ties or velcro.
Thanks. turns out the PDW Radboot 1000 comes with a rack light for free. Its battery operated which isn't my preference, but I like that it has a built in reflector and a lot of people on this forum seem to like it. The price is right so I may order one to try it. Worse case it can go on a backup bike or my wife's bike. Bonus is rack mount sounds like it would work with other PDW as well as Planet Bike lights.
Originally Posted by Paramount1973
The other light I saw as a good option is the Cygolight Hotshot. It is highly rated and you can bet a rack mount for $3-$8.