I use the strings "XXX" and "999" for this (the latter because you sometimes need a dummy value in a numeric context), and have a global git hook which stops me committing a changeset which includes them.

I occasionally need to override the hook, for example when using mktemp -t, or when some floating-point data actually contains a run of 9s. But mostly, it is quite specific at catching stuff that shouldn't be checked in.