Substituting the doc/<package> dir with a symlink

[Rodrigo Gallardo <rodrigo@nul-unu.com>]

Policy 6.6.4 says that, when unpacking an upgraded package

... A directory will never be replaced by a symbolic link to a
 directory or vice versa; instead, the existing state (symlink or not)
 will be left alone and dpkg will follow the symlink if there is one.

Which means that, when naively trying to replace the doc dir of a
package with a symlink to my arch indep -data package I get bugs like
#521047.

Is the 'right' solution to that adding a postinst check that deletes a
leftover empty dir and manually creates the symlink? Or will I get
into a worse mess? What do I do if the dir is not empty (which could
only happen if the dir was locally messed up with)? Do I just error out?