I don't know what you mean by rootstrap. But basically installing the hildon headers etc fills up the root. If it wouldn't fill it up why would some headers and dev stuff brick anything?

The chroot is interesting, that'll work for dev, since it's not needed for normal ops.

I'm trying to think quickly: if I bind mount dev proc etc, will I be able to chroot and apt-get install stuff? I should if I have a good root, OK that's what you mean by rootstrap Where can I download a good rootstrap? An identical rootfs with the clean N900 flash would be good.