There are plenty of usb debug cables based on FT2232 chips from FTDI, including the new OpenRisc USB debug cable developed by OrSoc. I think it would be quite interesting if this wonderful debug interface could support this kind of cable.
I've been using urJtag to drive my FT2232 based home made cable and current support is fair good. Maybe urJtag is good starting point.
Cable driver to support FT2232-based cables submitted by Jose Villar. Added to SVN and tagged for release in version 1.2.0.