Rails - Sep 2005 - Validations on instances

It would be nice to be able to:

@instance = Model.find 1
@instance.validates_presence_of :field
@instance.save

Of course, you shouldn''t do this all the time.  But it would be nice
to handle edge cases.  Another possible approach could be to
name/label validations in the class, and be able to disable them on
the instance-level.

Oh, yes I do know about single-table inheritance.  But there are cases
where I''d rather just modify the instance.  Anyone have any ideas for
hacks/workarounds?

Thanks,
Kyle Maxwell