Is there any point in updating fonts in rawhide to their Unicode 9.0 versions, before the change is implemented?
With Unicode 9.0 update libraries will be updated with latest UCD stuff.
Besides the font checking tools that do not know of Unicode 9.0 (or 8, I think), will anything important break after such an update?
There is no harm or even issues updating fonts prior to this change. Only issue that might happen is, particular character not get required support from harfbuzz and pango, since they do not know character properties and how to render them.