You could try something like this: label microcore
kernel /boot/bzImage
append initrd=/boot/microcore.gz quiet noswap tce=UUID=aaab6273-4a6c-4118-8eb2-e31a9b31edb3 waitusb=20:UUID=aaab6273-4a6c-4118-8eb2-e31a9b31edb3
..substituting your usb stick uuid and where "waitusb=20:UUID=.." means wait 20s or until device UUID=.. shows up on the usb bus.
Other than this, you could boot "base norestore" with your usb stick plugged directly into you machine and, once booted, remove the stick and reconnect behind the hub to check that it is recognised.