A new jack gotcha - 32 is not 24

[spm_gl]

To my dismay, I discovered that jack outputs 24bit in 32bit frames. So what, you ask? Well, my external DAC doesn't recognize 32bit. Many external receivers are quite happy with the 32bit frames, but not all. So now I need to knit an .asoundrc that converts the 32bit on s/pdif to 24bit. There is a plugin for that, but I've found no documentation on how to actually use it. We'll see, I guess.
Oh, and it is jack that's causing the problems. If I play a 24bit file directly with aplay, it works. Through jack, my dac sees a stereo input stream, but doesn't make a sound. The meters in envy24control dance happily to the music too.