Or you can use the u-boot bootloader[1], and boot a kernel and root filesystem from TFTP, right?
[1]http://www.denx.de/wiki/U-Boot
PS: Upon further inspection, PXE netbooting is already possible on the 3B under the name of "USB booting," it's just not the default in the hardware, and there might be bugs that are fixed in the new version.
And at this point, if you already use an SD card for the bootcode, might as well put the whole U-Boot onto the card (which I did)