RJP,
I believe the Xenarc PCs use a Sumicom motherboard, which has an integrated, on-board DC-DC PSU. It DOES require a REGULATED +12V supply. The CarNetix P1260 would be a perfect fit for this application.
However, from what I understand, the Sumicom mobo does not have a seperate ACPI header or input. I beleive one of my customers has soldered 2 wires across the "power" switch and attached them to a small relay, which is fed by the ACPI control signal out of the P1260. Check our forum. I think there are some details of the wiring there.
FYI....the MP3Car.com store will begin carrying the CarNetix P1260 very soon...
Good luck!