The only hangup (besides the fact that I'm an idiot and plugged the stereo into the wrong output port on the MythTV box) is that AirFoil is hardwired to transmit audio only on port 6000. Which any experienced Unix user recognizes as the place that X server :0 lives. This one-line change in /opt/kde/share/config/kdm/Xservers is all I needed to move the console X server to :1 and port 6001:
:1 local@tty1 /usr/X11R6/bin/X :1 vt7