It seems I've answered the question - I found the 135 training manual on my hard drive (don't ask) and it says that the low fuel sensors are on the fuel sensors in the supply tanks - so yes, if there was some sort of blockage between the mains and supply tanks, I guess the pilot could be faced with contrary indications of enough fuel to get home (from the quantity transmitters in each tank) and low fuel (the low fuel sensors in the supply tanks). Is that covered in the training?