I am also interested in this feature.
We use XCP-NG to run the virtual hosts in our DMZ. As you can imagine, the DMZ is completely separated from our internal network and the only way to transfer a file to a virtual server is to physically connect a usb drive to a host.
We use Xen Orchestra for everything except this. When we need to mount a USB drive we have to physically connect a laptop to the DMZ network and then attach the USB drive with XCP-NG Center.