You might be interested to know that I have just successfully 
cross-compiled a NetBSD kernel on my Linux system. (OK, you have
to 'make assym.h' on the NetBSD system. I'm sure there should be some
way of redoing assym.h not to require a binary genassym in the process.)
It involved a lot of hassle and a few tweaks to the Makefile (generally
replacing 'size' with '${SIZE}' and so forth to use the cross-tools.)
Oh, and bugfixing Linux' lorder(1). And installing and configuring
that strange version of make NetBSD uses...

I shall probably eventually stick some details on my WWW site 
(http://mnementh.trin.cam.ac.uk/), but I thought I'd see if there was
any demand for them before I went to the effort of writing things up...

Peter Maydell