on the 7c g3, when you unpack the boot.img,you will end up with one folder and 1 file, the zimage. and the ramdisk.
in the ramdisk are all the stuff that makes the tablet to work. (beside de kernel)
for example i found a rusian model of the arnova 7c g3, and same thing, no usb/adb working so i unpacked the boot.img, and i copied from the old boot.img the adb which is in sbin folder, then i copied also the file init.qcom.usb.rc that is ramdisk, which is in the ramdisk folder.
the init.qcom.usb.rc defines how the tablet is recognized in different moments when is connected to the pc. (usb debugg/mass storage/fastboot)
the system should be the same also for the other tablets, you only need to find out where is the ramdisk because from what i saw, usually you get only the kernel image when you flash some roms for this tablet.