pygrub, which is part of the xentools package, allows booting a Linux domU without having to keep a copy of the kernel and ramdisk in the dom0 file system.
What it does is extract the grub.conf from the domU disk, processes it, and presents a grub like menu. It then extracts the appropriate kernel and ramdisk image from the domU disk, and stores temporary copies of them. Finally, it creates a file containing:
This file gets processed by xend as if it were part of the domU config.
Most of the code required to do the NetBSD equivalent should be available as part of /boot or the libraries it depends on. The path to the source code for /boot is: sys/arch/i386/stand/boot.