I have a source tree.  I want to do a build from it and install the
  newly built system into a new disk mounted on, say, /mnt, such that
  adding a kernel and setting up my port's boot machinery gives me a
  bootable system.

See pkgsrc/sysutils/etcmanage.  This is way more than you need, but
it's a commented example of using build.sh in "BUILD-NetBSD" and
"INSTALL-NetBSD" does binary updates from a release (sets and kernels)
into the current running system.  I know you want to put this on a new
disk, but the existing script should serve as a good hint.  A patch to
add DESTDIR support to INSTALL-NetBSD would be welcome; I was just
thinking about putting current on a USB stick to debug ACPI suspend
without having to fsck my whole disk.

Basically,

  unpack a kernel
  unpack all the sets
  run installboot or whatever
  go into /dev and run MAKEDEV

I think that MAKEDEV can be run cross arch.