I did it!
It only took a full week of my spare time!
I'm going to help the NetBSD team update the official docs but in the mean time I'll briefly cover the working process here and maybe I'll update the OP soon.
To answer our previous question, no, installboot isn't required on the BPi. What IS requred is a kernel that has been patched to boot off wd0a instead of ld0a. A suitably patched kernel for 7.0 can be downloaded here:
https://www.netbsd.org/~tnn/bpi-wd0a/netbsd.ub
There is no need to install rsync to copy the root fs to HD, pax can be used for this. After mounting the HD under /mnt you'd run:
pax -X -rwpe / /mnt
BUILDING A CUSTOM KERNEL
$ cvs -z9 -d
anoncvs@anoncvs.NetBSD.org:/cvsroot co -r netbsd-7 src
$ cd src
$ vi sys/arch/evbarm/conf/BPI
# change the line:
# config netbsd root on ? type ?
# to:
# config netbsd root on wd0a type ffs
$ ./build.sh -m evbarm -a earmv7hf -j 4 -u -U tools
$ ./build.sh -m evbarm -a earmv7hf -j 4 -u -U kernel=BPI
Kernel will be located in:
src/sys/arch/evbarm/compile/obj/BPI/netbsd.ub
This kernel needs to be copied to the FAT partition of your SD card and fstab and boot.scr must be adjusted to boot off wd0a instead of ld0a.