xar 1.6 supports lzma as well as xz. Didn't ever use RPM5. As I read the docs, the xz-format itself is officially stable. Don't know how to implement support for new formats in TUnA, just thougt it could be as easy as replacing liblzma from LZMA-utils with the version from XZ-utils and add the xz extensions to info.plist...
If it's a heavier task to implement, don't hurry. Can easily be solved using Terminal.