I'm not sure I am totally following what you are trying to accomplish. Are you trying to bring in the value from another model as a new value on the table row and when edited, saves on the model associated with the table? If not, when that record gets saved, what are you hoping to have happen? The new value gets saved on the model not associated with that table but on the other record?
Is this what you are trying to do? If so, help to vote it up so our devs know how important of an issue this is without the hackery and custom field rendering that Pat described having had to use above.