Have you tried a Control- or a PageAdapter? I often use them for changing build-in EPiServer functionality. I'll see if I can get you some directions in a bit.
It seems to be \EPiServer\CMS\6.1.379.0\Application\UI\CMS\Edit\XFormsPostings.ascx that is responsible for the Form Data tab in edit mode. The grid control has ID='Grid', so I guess you could try creating a ControlAdapter (similar example), locate the Grid-control recursively in this.Controls and edit the item template; or see if there is something to be done to the objects in Grid.DataSource. If you use Ilspy or Reflector on EPiServer.UI.Edit.XFormPostings you will see what EPiServer does. The search, datasource assignment and databinding is done in the Search_Click event. Hope it helps.
Feels like there should be an easier way though, can't think of any..
//m
does any one know how we can ovveride the existing edit Panel Plugin "Form Data". I don't wanna to see all the posted data. I want add some more Fields and remove some is that possible.