Hi there Pablo!
- Buy a new chip with the original ASUS BIOS in order to
boot the system.
These pre-flashed BIOS chips are overpriced. You could download the latest BIOS from ASUS website and flash it directly to your existing BIOS chip using another computer and flashrom-supported hardware flasher.
- Externally flash the chip I have right now with a newer version
of coreboot.
RPi is a bit of an overkill: the cheapest flashrom supported programmer is USB CH341A which costs just a couple of dollars and perhaps is easier to use (since you don't have to worry about the configuration of onboard Linux and some other things). Check out the last part of this article - http://dangerousprototypes.com/docs/Flashing_a_BIOS_chip_with_Bus_Pirate - for more CH341A + flashrom instructions. If your BIOS chip is supported by flashrom, hopefully you could easily flash it following these instructions. The only difference is that the BIOS chip of KGPE-D16 seems to be a DIP-8 instead of SOIC-8, so you could insert this DIP-8 chip right in CH341A and SOIC-8 test clip isn't needed of course. Although there are DIP-8 test clips, if your chip is socketed - they aren't needed.
- The moderboard datasheet has a section called:
"Force BIOS recovery setting"
Of course this method relies on the functionality of proprietary UEFI, like Matt said.
Best regards, Mike Banon