Finally hacked together an implementation of Ephemerons for Squeak.
If you wonder what that is:
An Ephemeron is an object which refers strongly to its contents as
long as the
Ephemeron's key is not garbage collected, and weakly from then on.
Ephemerons
solve a problem which is commonly found when trying to "attach"
properties to
objects by using a registry. When some property should be attached to
an object, the property should (in terms of GC behavior) typically
have the life-
time that an instance variable of this object would have. However,
this is complicated by having an external association between the
object and its property such as:
registry ------------> association -------------> object
       |
       |
       |
       v
    property
Here, the registry (a third party) will hold onto the association
itself which
would require manual removal from the registry (instead of automated
garbage
collection). While this problem can be always be solved in any given
concrete
situation by using one of the various weak association types, choosing
the 'right' kind of association depends on a variety of factors some of
which
can change dynamically.
Ephemerons solve this problem by defining that the 'contents' (value)
of an Ephemeron will be held strongly until the key is known to be
garbage collected. From then on, the contents of the ephemeron will be
held weakly. Therefore, the contents of an ephemeron can only become
eligible for garbage
collection if and only if the key is garbage collectable which is the
exact behavior which we would observe for an instance variable of the
object.
The package is up at SqueakMap at
 http://map1.squeakfoundation.org/sm/packagebyname/Ephemerons
and will naturally require a new VM before the tests turn green.
Cheers,
  - Andreas