Apparently, the default SeaBIOS on the Chromebook Pixel only exposes 1M of RAM.
To boot a kernel and initramfs, you need a bit more than that. Here's how I calculated how many megabytes the on board Intel graphics card removes from the main pool of RAM.
Boot (anything, but I used sysrescueCD) using "mem=1G" kernel parameter. You need to remove everything right of and including the "--". Otherwise the kernel will ignore those arguments.
Run a program such as "free" or "htop" to find out how much RAM the system actually has.
Which means that (1 * 1024) - x = 656, so x = 368M is used by the graphics card.
The Chromebook Pixel has 4G of RAM. so (4 * 1024) - 368 = 3728.