Please test, and provide lots of feedback, cross compiling RPMs will be a major step forward! I hope to try some more packages this weekend, if time permits.
Great work, Keith. About that --ignoreos: wouldn't this: http://www.rpm.org/max-rpm/s1-rpm-multi-build-install-detection.html help? It seems rpm checks architecture against 'uname', so it may fail right in this point -- there are some directives to define build and compatible platforms.