Other things you may want to experiment with is gen_init_cpio.c from linux kernel tree. It makes creating initramfs file structure easier from scripts.

And finally if sys/isolinux is also fun to use for minimal boot images.

If it's just a PoC, then:

1) I remember seeing a linux firewall/gateway set up to run with just the kernel, without any userspace at all. Completely unhackable.

2) To print some text or run a simple program, I belive DOS without a memory manager would be even faster.

3) It takes 1s to boot linux, but an ordinary PC takes 10s to get to that linux. Even U-boot on ARM takes some seconds to load a kernel.

BTW, if anyone knows any current platform that can XiP a linux kernel, please share.