Andreas Stieger (SUSE, SVN) has written a pre-commit hook script which rejects commits of shattered.io style PDFs
https://svn.apache.org/viewvc/subversion/trunk/tools/hook-sc...
This is the first mitigation available. If you are responsible for an SVN server at risk, please make use of this hook.
If somebody could make a similar hook for Windows and post it here or to dev@subversion.apache.org that would be highly appreciated.
(edit: switched script link to HTTPS)