When I've flashed with Linux, I haven't had to hold down the power button at all. Steps:
(removing battery is optional)

- Turn off the unit
- Remove battery and reinsert it
- Connect USB to PC
- Run flasher (it will say "Suitable device not found...") Look here for more info:
http://maemo.org/maemowiki/HOWTO_Fla...ImageWithLinux
- Plug in power adapter
- Now the device is recognized and the image download should begin. The 770 should also power up on its own at this point.

Have you *ever* been able to flash the device, even when it was healthy? If not, perhaps the problem is with the USB setup on your PC.