There are also tools intended for bootstrapping & cross compiling distros.
Scratchbox1 (SB1) & Scratchbox2 (SB2) are such beasts. First one requires maintaining your own built tools "distro" which can easily bitrot & get out of sync with your distro, the latter can map tools to wherever you want to, typically this would be a directory containing x86 versions of your target packages (e.g. x86 debootstrap when cross-building Debian).
If your x86 host distro is the same one that you're trying to build for some other architecture, using SB2 so that it maps missing things to your host and letting everything else be run with Qemu, makes it much easier to build bootstrap stuff.
MeeGo has also some support from cross-compiling RPMs, but that's mainly to accelerate building, not AFAIK for bootstrapping purposes.