That's how I do it, save that I don't bother with OBJMACHINE; I just use
BSDOBJDIR as a local filesystem on each of my build systems. I mount
/usr/src rw and "make obj", then remount /usr/src as ro, and "make build".

The nice thing about building with ro /usr/src is that when someone botches
a Makefile, this will catch it, and the tree doesn't suffer (much).

	Erik <fair@clock.org>