I worked through a few more modules. Still really enjoying the degree of focus I get from knowing that each test module is only responsible for one source module. I've had fix a few howlers of bugs as a result of, just, it turns out I never tried running most of the new code. As such, I've had to make code changes, with varying degrees of gracefulness.
I've got it up to about 60% coverage now, and I'm clearly hitting code that was never exercised before. The tests I have written at this point are probably sufficient to get extremely high coverage just by pasting them around indiscriminately, but I'd prefer to paste them around discriminately.
I am spacing out on anything else to add, so I guess I should stop.