More research has indicated that you just can't do Xen PCI passthrough when Solaris is involved. So my dreams of running a file server and myth backend on the same machine are shattered. At least I learned a lot about Xen (and have Linux running VMWare running Xen/Linux running Linux...).
With a 64-bit PCI SATA controller and a bigger case, I could use my current linux machine, atlas, as a Solaris file server. Unfortunately, nobody seems to make such a controller without including $200 worth of useless RAID hardware.
The myth backend and all of its tuners would have to move to my new desktop.