USB does not have DMA capabilities. If you have access to DMA, you have already 
got access to the controller or the usbvm.

You probably can get into USBVM easily from an USB device by logging as root on 
the login screen. This, however, assumes that keystrokes are not captured by 
other means, which I am not sure if it is true on the latest Qubes version, 
since some input proxies have been implemented. On 3.0, I was able to shutdown 
Debian USBVM by ctrl+alt+delete, which suggests that some more complex attacks 
(using the default empty root password) might be possible on this version. On 
newer version, I haven't tested it.

Nevertheless, I have disabled all USB keyboards on my USBVM for the reason 
above. They are enabled only in dom0, which uses a separate USB controller.

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