git fat share object store must be read-write

[afenkart]

I checked out 2 large repositories (70GB) twice on the same machine, but under different user accounts.

I'd like to share the git-fat object store between the two accounts, but only should have write access to
the objects. Using a symbolic link as suggested here, does not work:
https://github.com/jedbrown/git-fat#implementation-notes

Is there a way to add some kind of git-fat-ref link to the shared store, while using a local writable folder for the clear/smudge operations?

git status:
error: cannot feed the input to external filter git-fat filter-clean
error: external filter git-fat filter-clean failed 1
error: external filter git-fat filter-clean failed
Traceback (most recent call last):
File "/home/debugger/bin/git-fat", line 515, in
fat.cmd_filter_clean()
File "/home/debugger/bin/git-fat", line 255, in cmd_filter_clean
self.filter_clean(sys.stdin, sys.stdout)
File "/home/debugger/bin/git-fat", line 214, in filter_clean
fd, tmpname = tempfile.mkstemp(dir=self.objdir)
File "/usr/lib64/python2.7/tempfile.py", line 304, in mkstemp
return _mkstemp_inner(dir, prefix, suffix, flags)
File "/usr/lib64/python2.7/tempfile.py", line 239, in _mkstemp_inner
fd = _os.open(file, flags, 0600)