Thanks for the thoughts, Exeric! I've been considering putting the transformers into the crawl space - it should be easy enough to mount them directly under the wall the switch is in, drill a hole upwards, and run the wires through there! For retrofitting the hallways, I might do exactly that.
For the kitchen, I've put the transformer about 1 1/2 ft below the switch on the other side of the wall, into a junction box in the wall in a closet. The supply wire comes from the switch box, is spliced off down into the new box that houses the transformer, and 12v come back up into the switch. For the fixtures (one is finished) I'm just using the existing Romex wiring and extend it from the existing ceiling box to the fixture:
Of course from the ceiling box to the fixture needs to be wired properly - I need to run the 12V wire through the ceiling a short distance because the existing ceiling box isn't in the location I need it in. Darn builders, and their habit of not considering the outlandish modifications I might someday plan on!
For the kitchen fixtures I decided to go with self-adhesive LED strips instead of modules, because I wanted a higher density of LEDs (and higher output along with it). I picked one with 600 LEDs @ 2A for 5 meters (
http://www.ebay.com/itm/220768031684...84.m1439.l2649), and ran three strips of a little less than 3 feet in length, for a total of about 350LEDs, so on the order of 1.2-1.3A (should around 15W or so).
The single fixture shown here throws about as much light as a single one of the original 75W fluorescent fixtures, maybe a little less - but since it's more directional downwards, the work space is nicely illuminated. I need two more of these (a little shorter) for a total of about 40W or so, replacing the three original fixtures at 225W combined. I don't understand why this sort of thing isn't done everywhere all the time in new construction.