I just got a reply email from the developers of the Systat dashboard widget that they plan to put a temperature monitor in a future version. I know this info is available somewhere else, but I forget where. I don't have a PowerMac yet to work with.
I have not checked into what "standard power connectors" inside the Mac are available, but I found I've already got an old 5 volt relay I could work with which would match the power available on a USB Port. Otherwise I would have to bring power from the internal connector to the outside of the computer. This may be easier than I expect, easier than the relay.
The fan would need to fit behind a 4" x 10" grate at the top/front of the cabinet in a way that air would be forced from there through a similar grate behind the power mac with 2" clearance, then beneath the cabinet to a grate at the bottom/front of the cabinet. That would allow room for two 120mm fans. I understand a dual power mac has to work hard (and presumably hot) to decode full size HDTV.
I'd have to double check all the details, but either with internal standard power, or a USB relay, it appears I could make it work.
Anyone know where I can find documentation on internal standard power connectors?