The boost builder (jam) goes into an architecture dependent sub-directory after being built. When it came to running it, the present ebuild (boost-1.29.0.ebuild) hardcoded the directory to the x86 version. Just by using ${ARCH} in the directory name, the ebuild works on ppc, and perhaps others - I didn't check whether gentoo architecture names matched every boost suffix.
Created attachment 7212 [details, diff] Fix to allow building on ppc. Apply this patch to boost-1.29.0.ebuild and you should be able to build on ppc.
tossing this to nall toget some attention, my workload is _way_ overboard ATM
k, i was able to build on ppc successfully. i'll make sure it still works on x86 tonight (i can see no reason why it wouldn't) and if so, i'll patch the build and mark it ~ppc.
this is fixed and committed in CVS.