Just an update: libxml2-16, libxslt and libxmlsec1-1 have all migrated. There are a couple of binNMUs still building, but now it's just a matter of fixing or removing the remaining packages.
For the record, I ended up working from Matthias' .ben file (including the .build-depends regex) as it appeared to deal with dependency levels better. I did not schedule binNMUs for any of the 'unknown' packages, as these had neither a dependency on the old libxml2 library nor the new libxml2-16 library.
