Afaict you could do a "source amd64 arm64 armel armhf i386 mips mipsel powerpc ppc64el s390x" upload to unstable so that binaries for all release architectures were supplied by you rather than by buildds.Hmmm, this is what I missed. :-( I guess the only chance is to uploadto t-p-u, right?
It would be a PITA to do (build all the binaries, bring them back to a single box and mergechanges them into a single upload) but i'm pretty sure it would work.