Just to beat the dead horse more (blasted horse), I don't see why you couldn't build a somple one-shot timer that pulls the power-on connection low for a few seconds whenever power is connected.
Something like a 74HC123 would be ideal. Hell, it could be offered as a simple plugin module that connects between the main board and the daughterboard, and produced for something like 20$ in medium quantities (50 - several hundred).
Personally, I bought the chumby as an "Alarm clock that also does other things", mostly because back when it was released (Last christmas), that's what all the advertising claimed it was. You can retroactively claim "that's not what we meant", but it doesn't change the fact that a large portion of the userbase bought the chumby predicated on that idea.
Fortunately, I have reliable power where I live, and if I'm late for work once a year, no one cares.
I'l think about design later tonight.
Edit: whoops, the power switch is really a pull-up. Still you could easily OR a one-shot and the power switch with a couple of diodes.
Edit: Further hardware questions...
On the schematic: http://files.chumby.com/hdwedocs/Rev37_release.pdf what type of device is D10?
It looks like a transient voltage supressor (E.G. Tranzorb), but it's not specced.
The P33VBKUP Should be able to source enough power to run a lov voltage 1-shot, and with the two seperate timers in a 74HC123, you can cascade them to allow the chumby enough time to start up before signaling a power-on.