Assuming cd image is in the working directory,
qemu -hda /dev/domUs/centos5 -hdb /dev/domUs/centos5swap -cdrom ./CentOS-5.4-i386-bin-DVD.iso -boot d -vnc 0.0.0.0:0
Use a VNC client (ie. RealVNC) and connect to the display – [Hostname or IP Address of the Xen server]:0

Seems to be bloody slow, but working. For some reason, restarting it after the install causes it to bomb out. Xen doesn’t like the kernel (despite it being customized for Xen – or at least, it seems to be, because I selected ‘Virtualization’ in the install options) and QEMU bombs out with “Failed to gain physical IRQ nn”. To which I have to say, “no shit Sherlock. You’re supposed to *virtualizing* the stupid thing, not giving it access to the physical hardware.” >.>