With dpkg from experimental and the new zlib upload in unstable, I've now got a partial Multi-Arch install. There are more packages necessary, particularly related to how -dev packages can exist and how a cross compiler gets built/installed in Multi-Arch world. One bug #659588 in libglib2.0-0 so far but that's quite good seeing as it's been all but impossible to test the Multi-Arch changes in existing packages until now.
$ dpkg --print-foreign-architectures
$ dpkg -l | cut -c -80 | grep armel|grep -v cross
ii gcc-4.6-base:armel 4.6.2-14
ii libc6:armel 2.13-26
ii libdatrie1:armel 0.2.5-3
ii libffi5:armel 3.0.10-3
ii libgcc1:armel 1:4.6.2-14
ii libgmp10:armel 2:5.0.4+dfsg-1
ii libgmpxx4ldbl:armel 2:5.0.4+dfsg-1
ii libgomp1:armel 4.6.2-14
ii libmpc-dev:armel 0.9-4
ii libmpc2:armel 0.9-4
ii libmpfr-dev:armel 3.1.0-3
ii libmpfr4:armel 3.1.0-3
ii libpcre3:armel 8.12-4
ii libpixman-1-0:armel 0.24.4-1
ii libpng12-0:armel 1.2.46-4
ii libpopt0:armel 1.16-3
ii libpwl5:armel 0.11.2-6
ii libselinux1:armel 2.1.0-4.1
ii libstdc++6:armel 4.6.2-14
ii libthai0:armel 0.1.16-3
ii linux-libc-dev:armel 3.2.4-1
ii zlib1g:armel 1:1.2.6.dfsg-1
There are more packages which can be installed on amd64 for i386 as #659588 only affects Multi-Arch versions which cannot execute compiled binaries from the foreign architecture.
Further progress, inside a test chroot, involves using gcc-4.7 from experimental but even then, libc6-dev is not installable as a Multi-Arch package. That's the current blocker for toolchain stuff.
ii gcc-4.7-base:i386 4.7-20120210-1
ii libgcc1:i386 1:4.7-20120210-1
Once we have libc6-dev:armel installable on i386 and amd64, work on cross-building Emdebian can be considered again. It's been a long time.