Polymorphic belongs_to with :class_name => abstract class

SUMMARY: How do I assign objects, of different subclasses of an abstract
class, to a belongs_to relation?

I had the following class hierarchy:

Location < ActiveRecord::Base
   Address < Location
   Building < Location

and a class Person with these relationships:
   has_one :address, :dependent => :destroy
   belongs_to :building

Location is backed by table "locations," and Person by "People."

I had to refactor to insert an abstract class between Location and
Building:

Location < ActiveRecord::Base
   Address < Location
   SharedLocation < Location
      Building < SharedLocation
      DeletedLocation < SharedLocation

I now want Person#building to refer to either a Building or a
DeletedLocation. Refactoring the foreign key and accessor would be a
major pain.

I changed the relation for Person#building to:
      belongs_to :building, :class_name => 'SharedLocation',
:foreign_key => 'building_id'

(:foreign_key because I got a deprecation warning on specifying
:class_name without one.)

I also moved
     has_many :people, :dependent => :nullify
from Building to SharedLocation. In the course of flailing about for a
solution, I added ":foreign_key => 'building_id'", but I don't know what
I'm doing.

I ran my unit tests for setting the relation, and am now seeing
failures/errors, all of which seem to boil down to this: Attempting
      person = Building.new(...)
gets an AssociationTypeMismatch, "SharedLocation expected, got
Building."

How do I make belongs_to relations polymorphic?

Rails 2.3.2
ruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin10.0]