If I empty out my TMPDIR, move sstate-cache to sstate-cache-orig and then set SSTATE_MIRRORS to point to that directory as follows:

SSTATE_MIRRORS = "file://.* file:///home/paul/poky/poky/build/sstate-cache-orig/PATH"

Then I run bitbake quilt-native I see the following:

----------- snip -----------
Checking sstate mirror object availability: 100% |########################################################| Time: 0:00:00
NOTE: Executing SetScene Tasks
WARNING: quilt-native-0.64-r0 do_populate_lic_setscene: Task do_populate_lic_setscene changed cwd to /home/paul/poky/poky/build/sstate-cache
NOTE: Executing RunQueue Tasks
NOTE: Tasks Summary: Attempted 9 tasks of which 1 didn't need to be rerun and all succeeded.
----------- snip -----------