My experience with cross-compiling NetBSD was just doing the kernel. That was maybe ~ 7 years ago. At the time, I wrote a guide on how to do that, but never posted it. Looking at it again there are a couple of references that might be useful:
Cross-compiling NetBSD with build.sh (from the online Guide)
build.sh: Cross-building NetBSD (PDF by Mewburn & Green)