From https://lists.oasis-open.org/archives/office-comment/200903/msg00155.html :
Instead of taking svg NCNames & cherry-picking / redefining their
semantics, include e.g svg tiny wholesale, and add necessary
extensions according to
https://www.w3.org/TR/SVGTiny12/extend.html#ForeignNamespacesPrivateData
Note that this is very likely backward-incompatible.