I have written an add-in so I can be of some help to you.
Code:
Public VB As DTE2
Dim PI As ProjectItem
Dim EP As EditPoint
Dim TD As TextDocument
Dim obj As Object
Dim Win As EnvDTE.Window
PI = VB.Solution.FindProjectItem("Form1.vb")
Win = PI.Open(vsViewKindTextView)
Win.Visible = True
PI.Document.Activate()
obj = VB.ActiveDocument.Object("TextDocument")
TD = CType(obj, TextDocument)
EP = TD.CreateEditPoint
With EP
.FindPattern("<summary>")
.LineDown(2)
.Insert("'Comment goes here")
End With
I took some code and tried to cobble something together. I haven't tested it, but was trying to give you an idea of where to start. I will help anyway I can, but there is not much out there on this subject.