I'm giving a talk at Usenix in Santa Clara in 2 weeks on u-root, and I'll be setting up a Plan 9 BOF (assuming there is room) with a difference: we're going to demo the GPL'ed Plan 9 code base booting on a small amd64 cluster (assuming it's working by then!). The system is built with gcc, although some of us are advocating moving to clang. A full build of all libraries, binaries, and kernels takes about 2-3 minutes. It's now possible to debug the kernel, including setting breakpoints and examining structures, with gdb when you are running under qemu. If you're at Usenix drop on by and see what you think.
If you want to know more you can contact me off-list.