You could try to get your desired output by making few changes in fields_view_get().  (It is similar to a hack) 

 from lxml import etree
 from openerp.osv.orm import setup_modifiers
 class product_template(orm.Model):
    _inherit = "product.template"
    def fields_view_get(self, cr, uid, view_id=None, view_type=False, context=None, toolbar=False, submenu=False):
           if context is None:
               context = {}
           res = super(product_template, self).fields_view_get(cr, uid, view_id=view_id, view_type=view_type, context=context, toolbar=toolbar, submenu=submenu)
           if view_type == 'form' and self.pool.get('res.users').has_group(cr, uid, 'YOUR_GROUP_ID'):
               doc = etree.XML(res['arch'])
               method_nodes = doc.xpath("//field")
               for node in method_nodes:
                   node.set('readonly', "1")
                   if node.get('name', False) and node.get('name', False) not in ['name','image_medium']:  #Add fields to skip readonly
                       setup_modifiers(node, res['fields'][node.get('name', False)])
               res['arch'] = etree.tostring(doc)
           return res

CREATE 3 groups
YOUR_GROUP_ID
YOUR_GROUP_CHILD1_ID  --> inherits YOUR_GROUP_ID group
YOUR_GROUP_CHILD2_ID  --> inherits YOUR_GROUP_ID group

You can check whether the user has access for a particular group by doing this
self.pool.get('res.users').has_group(cr, uid, 'YOUR_GROUP_CHILD1_ID')