I didn't figure out how to do this client-side, but I am able to set the radio buttons server-side.

Using the following code I was able to set the radio button in code-behind using C#:

WebDataGrid grid = WeldsDataGrid;
RadioButtonList rbl_response;

foreach (GridRecord row in grid.Rows)
{
   rbl_response = (RadioButtonList)row.Items[4].FindControl("rblStatus");
   rbl_response.SelectedValue = "Complete";
}