Aprilaire 700 install no outlet for 120vac hookup?
Alright -- while that box looks pretty stuffed, best I can tell, it has enough room for a couple more wires (being a big ol' 4-square with a mudring to boot).  If you can get an EMT conduit run from where that box is to where you're sticking your humidifier (half-inch'll do quite nicely as all you're shoving through it are 2 14AWGs) and some black (or some other color than white, grey, or green) and white 14AWG THHN run through it, you're 90% of the way there.

From there, just get a self-grounding, single, 15A receptacle (not a duplex receptacle) and whack it in a surface mounted single gang "handy box" attached to the wall -- black to brass, white to silver, and let the EMT be your ground.  In the ceiling box, with the power off of course, cut the orange wire in two, strip the ends, pull some slack out on the orange wires, connect your new black to the cut ends  of the orange wires, and connect the new white in with the big bundle of white wires. Once everything's hooked up, flip the power back on, and you're good to go.

This arrangement is allowed by 422.12 Exception 1:


  422.12 Central Heating Equipment. Central heating
  equipment other than fixed electric space-heating equipment shall be supplied by an individual branch circuit.
  
  Exception No. 1: Auxiliary equipment, such as a pump,
  valve, humidifier, or electrostatic air cleaner directly associated with the heating equipment, shall be permitted to be
  connected to the same branch circuit.
  
  Exception No.2: Permanently connected air-conditioning
  equipment shall be permitted to be connected to the same
  branch circuit.