Following Marcus Meissner's advice how to build a WoW64 setup (thanks!), I have succeeded in building a set of co-installable i386/amd64 Debian packages. Those won't make it into the "wheezy" release as I was just a bit late for the freeze, but I have uploaded packages to experimental.
What didn't work was building the 64-bits of wine on Debian/kfreebsd-amd64 (Debian userland including GNU libc on a FreeBSD kernel): https://buildd.debian.org/status/fetch.php%3Fpkg%3Dwine%26arch%3Dkfreebsd-amd64%26ver%3D1.4.1-1.2%26stamp%3D1341358832
Apparently the reason is that the uc_mcontext structure does not store the DS, ES, FS, and GS registers. After I made the changes below, everything compiled, but how useful is the result? Are those registers contained in uc_mcontext on "plain" FreeBSD?
Cheers, -Hilko