Acer TravelMate 4720 - Battery Charges but No Power-Up

Was given an old TravelMate 4720-6220 circa 2007 that's in fairly nice condition but won't power up so just asking for ideas. When plugged in, the battery charges correctly - I watched it do a full charge and the light switch from orange to green when finished - but the power button gives no response at all, both on and off AC and with/without battery installed. That being said, when you plug it in initially, it briefly powers on by itself - green power light comes on, monitor lights up no video is displayed, then shuts off or goes into charge mode if the battery is connected. There are no beep codes during this and the power button does nothing. Things checked/tried:

* Removed RAM, wi-fi card
* Checked and re-connected all cables and drives
* Checked board for corrosion or obvious signs of failure
* New thermal paste - here I noted that there was a pretty heavy buildup of dust 'n' crud between the fan and heatsink radiator. I cleared all this out of course, but it was enough that I expect the original owner probably experienced some heat-related issues at some point. The original thermal paste, however, wasn't exceptionally cooked and looked basically like what you'd expect on a machine this age.
* Replaced CMOS battery
* The power button on this model is a physical button mounted on the motherboard itself, it appears undamaged.

Any ideas? I don't really care one way or the other about the thing, but if the symptoms sound familiar to anyone I might as well give it a shot. Thanks.