Should charge just fine, as I also use a USB battery pack that recharges devies, and it puts out 500mA. As for the Atrix, it was reported as fully working using openwatch 1.5.0.
Here is the compatibility list of what has been confirmed as working and what has been confirmed as not working.
http://www.getinpulse.com/hack/forum/viewtopic.php?f=5&t=165