A friend with a Zaurus pointed out OpenEmbedded (more detailed blurb) - a project to produce a more coherant build system than the mess of customised buildroots that are currently used for embedded Linux development. At the moment the focus appears to be on ARM PDAs, but they do mention that they're interested in supporting router development. The system was designed to support multiple target and host architectures cleanly - even Windows eventually - and to look like something between Gentoo's Portage and the current buildroot system they've been using for Open Zaurus.
Perhaps this would be worth looking into as a build system for future versions of OpenWRT? Less duplication of effort is always nice, and buildroot is just a little hairy.