Bringing new 12/2 circuit to old knob/tube switches, but do not see a neutrals anywhere. Hidden?
Probably a shared neutral which was common with K&amp;T.  Like you said, there could be one neutral in the attic, and you only see hot wires being switched.  The fixtures will all be connected into the shared neutral.  You might be able to see this in your new panel, because your neutral bus will have fewer conductors than there are circuits!  You'll could also notice if you shut off your breaker to the circuit you think you are working on, the neutral will still have current running through it when someone turns on a light switch that is on a different Knob and Tube branch! Ouch!  

If this is the case, you should just rip it all out and do it right if at all possible.  Especially for bathroom you need to run have GFCI on all your circuits to be up to code and to be safe.