Question: I plan to add some lighting in positions on the ceiling that doesn't have nearby wire available.

The there is no free space between the ceiling/roof in which to run a wire (there's the wood, then solid foam block insulation, then weather barrier, then the shingles). So, I would need to run wire (12/2 or 12/3) to that location somehow. I'm aware of Wiremold, but that wouldn't look very good with the style and material of the ceiling itself.

Is it possible, instead, to run the wire through a 0.5" channel routed out of a 1x1" piece of wood and affix that wood to the ceiling? I read through California electrical code (I'm in San Bernardino County, California), but didn't see anything specific around this (unless I missed it, or interpreted it incorrectly).

Thanks in advance.

Answer: 
If your foam block is multiple layers deep sometimes it's possible to run a fish tape between layers. Take off the leader if present, use a blowtorch to heat the fishtape and fold about 3"(75mm) tightly back on itself (no loop), tape tightly together starting 1" down from the tip, stretching the tape slightly and use only 1 layer if possible. Use a file or sandpaper to smooth the tip if necessary. 1/4" or 3/8" fish tape is better for longer runs. If the fish tape is curly, straighten it first with a fish tape straightener. Take care and use both hands to feed the tape in, one hand pushing from about 18"(450mm) away and the other hand close to where the fish tape enters to redirect the force. For long hops, especially if you have to fish through partial obstructions, a toner is useful to locate the fish tape in the ceiling. If you get it close but a small amount (<3f(1m)) to the side of the destination, usually you can cut a hole at the destination and hook it over with a second fish tape.

If you can get a fish tape through, usually an armored cable can be drawn in. You can untape the head to use as a pull loop (don't try to unbend the tip, just cross the wires through the loop and fold back 4-6" and tape tightly from 2" onto the armor up to 2" past the folded back fish tape and back, 2 layers to reduce friction. The armor should be stripped enough that the folded back wire does not pass over armor). Note before starting that armored cable pulls nicer in one direction. The armor coil should end on the outside on the end you are feeding in. The person feeding the cable should feed as described above, and the person pulling should do the reverse, with the far hand providing pulling force and the close hand redirecting the force to pull flat between the sheets rather than down into the soft material below.

You may need an electrician to do this for you though depending on where you live.

Also consider that wiremold can be refinished many ways to be more aesthetically pleasing. Real wood edge laminate would probably give you the look you want. For short hops and hanging fixtures you can also consider hanging fixture wire strung through nice chain.
Can't say 100% certain with respect to code, but I did something very similar, with agreement of my electrician. He fished NM cable from bathroom lighting up to attic and through a hole to peak of ceiling between kitchen and dining room, around 8 feet along ceiling, then into ceiling to get to middle of dining room for new chandelier. I covered the 8' section with a piece of molding from Home Depot and it looks like it was always there.