"deprecate" a column

Does anyone know of a plugin to "deprecate" a column so active record won't try and save the column back to the DB? This would allow the old column to eventually be dropped without having to take down your site or stop mysql replication. For example:

You may be able to modify this plugin... it says it does half of what you want.. (the save part, not the create part):

http://www.railsify.com/plugins/61-validates-constancy

-philip