Cheap Windows driver signing for 64 bit Windows 7
No, the driver has to be cross signed by one of those specific certificates and thus the driver has to be signed by one of those CAs. You can disable driver signing on the machine for testing purposes, but obviously you don't want to do this on production machines. Sorry, that's just the way it is.
LibUSB_win32 is now already signed, according to http://sourceforge.net/apps/trac/libusb-win32/wiki
It says: "Vista/7/2008/2008R2 64 bit are supported from version 1.2.0.0 since a Microsoft KMCS accepted digital signature is embedded in the kernel driver libusb0.sys."
So the only thing you have to do is update your libusb_win32 driver.
To allow loading into kernel, you have to sign with those CA and have WHDL checked.
The only alternative would be using the user mode driver framework. (but libusb does not support it -- it was discussed, but never implemented)