That later turned out to be today, and after about 2 hours of various fussing about I discovered that the reason the dork had used brake cable jacket for the rear derailleur is that he had accidentally swapped the grommet for the rear brake with the one for the rear shifter. So I swapped them back to their correct locations and used a spare piece of shifter cable jacket as it should be.
Next up I had to unthread all the inner cables so I could clean up the spaghetti mess at the handlebars. Conveniently the shifter housings turned out to be the right length once I had swapped them to the correct crossed-routing, but the brake cables needed to be cut down by about 2 inches. A simple but slightly tedious job. Luckily rerouting the inner cables turned out to be relatively painless, though I did end up replacing the shifter inners as the ends ended up a bit frayed and were being a bit too cantankerous to reuse. They're cheap and I had spares, so whatever.
The only fuss with routing the cables turned out to be getting the rear shift cable down the chainstay, and I ended up pulling out the crankset to give me a bit more room to work down at the bottom bracket. It was then that I discovered that A: the crankset was only bolted together hand tight rather than the 50nm specified torque, and B: the bearing preload had not been set at all, leaving about 1mm of slack for the cranks to shift left and right. Great. Thankfully both of these things are easy to correct, but it would have been pretty ugly had I not noticed it.
As of right now the only work left to do is clamp down all the cables, adjust the shifting, and wrap the bar tape, but I felt like taking a little break. I'll probably get back to things a little later this afternoon, so you can look forward to seeing the photos then (I hope).