So, the firewall works well. I disconnected the fan, and now it's totally silent. My powermac is loud, but I never realized how noisy the old firewall was.
Now APM and ACPI are kicking me around. The motherboard I bought has 3 onboard RealTek 8139 10/100 ethernet chips; all have the Wake-On-Lan feature. So (and I must admit this is the first time, so why should it work?), I turn the device off, send it the magic packet and...nothing. Hm.
There is no section of the BIOS (Phoenix) specifically to enable WOL, and since the chips are on the board, there is no WOL cable to connect to the motherboard.
So, I search Google for "linux wake on lan onboard ethernet" and turn up 3 pages I cannot read. Either this is a very easy problem to deal with, or I am the only person in the entire world to have taken it on. Somehow I never believe in the second scenario.
Intel has a page on this, but I was too sleepy to read it tonight. I'll have another go at it tomorrow.
Added 25 Feb:
Fanless, the appliance runs at about 37-40C. The BIOS reports that the CPU runs at about 39-42C. That is acceptable. The average power draw is 8 watts. I've used the acpi modules to script a shutdown. When you hit the power button, it boots. Hit it again, and it shuts down cleanly. The machine boots faster if I run the / filesystem read-only and