I might be late to the party but i think you can use https://apps.openerp.com/apps/modules/8.0/message_post_model/ to solve your needs or you can dig deep at how they've done it by looking at the source code.

you can override just track_visiblity attribute :

name = fields.Char(track_visibility='onchange')

In the big comment atop class Field in fields.py, I noticed "For instance, the second class below only adds a tooltip on the field...."  I wanted to see the lead names, aka subject lines, in the lead notifications, and I was successful like this:

from openerp import models, fields

class crm_lead(models.Model):
     _inherit = 'crm.lead'
    name = fields.Char(track_visibility='onchange')

You do need the overhead of getting the datatype correct for the field.
Then the track_visibility attribute can be changed free of further overhead.