It would not be simpler to do a patch to interpret differently the soft link, kind of: "if
sticky bit is set, the target file is read-only, and the soft link is itself writeable, when a
modified file is saved, the symlink is deleted and the content of the file is itself written
in the filesystem".
That would imply the filesystem supports soft symlink and sticky bits, but would not be that
intrusive, and you can still have read-only files and manage a chmod on this "unionfs".
man chmod extract:
chmod never changes the permissions of symbolic links; the chmod system
call cannot change their permissions. This is not a problem since the
permissions of symbolic links are never used. However, for each sym-
bolic link listed on the command line, chmod changes the permissions of
the pointed-to file.