Try overriding your renderer''s OnClick method. In the override, check where the mouse position is, and if it is close to your button, then show your popup. To do this, you probably will have to expose the PopupContainer and Button so the renderer can access them in some manner (maybe make them public or add public methods to handle it).
Protected Overrides Sub OnClick(ByVal rowIndex As Integer, ByVal colIndex As Integer, ByVal e As System.Windows.Forms.MouseEventArgs)
MyBase.OnClick(rowIndex, colIndex, e)
Dim userCtl As CALDatePicker = Me.Control
Dim pt As Point = userCtl.PointToClient(Control.MousePosition)
Dim rect As Rectangle = userCtl.btnImage.Bounds
rect = New Rectangle(rect.Left - 20, rect.Y, rect.Width + 20, rect.Height)
If rect.Contains(pt) Then
userCtl.PopupControlContainer1.Focus()
userCtl.PopupControlContainer1.ShowPopup(Point.Empty)
End If
End Sub