I had the same problem and found a way to do it using a custom summary type. Essentially when you implement the ICustomSummaryCalculator, add a constructor to the class and pass in the grid by reference. Don't do anything in the AggregateCustomSummary method because it only sees the visible rows. Instead do a loop through of the passed-in grid in the EndCustomSummary method. This makes your grid available to your custom class and you can do anything with it inside the custom calculator.

Public Class MyCustomSummary
    Implements InfraWinGrid.ICustomSummaryCalculator

    Private m_pGrid As InfraWinGrid.UltraGrid

    Public Sub New(ByRef pGrid As InfraWinGrid.UltraGrid)
        m_pGrid = pGrid
    End Sub

    Public Sub BeginCustomSummary(ByVal summarySettings As InfraWinGrid.SummarySettings, ByVal rows As InfraWinGrid.RowsCollection) Implements InfraWinGrid.ICustomSummaryCalculator.BeginCustomSummary
    End Sub

    Private Sub AggregateCustomSummary(ByVal summarySettings As InfraWinGrid.SummarySettings, ByVal row As InfraWinGrid.UltraGridRow) Implements InfraWinGrid.ICustomSummaryCalculator.AggregateCustomSummary
        'Do nothing
    End Sub

    Private Function EndCustomSummary(ByVal summarySettings As InfraWinGrid.SummarySettings, ByVal rows As InfraWinGrid.RowsCollection) As Object Implements InfraWinGrid.ICustomSummaryCalculator.EndCustomSummary
        Dim intPos As Sys.Int32 = summarySettings.SourceColumn.Index
        Dim decSum As Decimal

        If intPos >= m_decONUninvested.Length Then intPos = m_decONUninvested.Length - 1

        For Each pRow As InfraWinGrid.UltraGridRow In m_pGrid.Rows
            If pRow.Cells(intPos).Value IsNot Sys.DBNull.Value Then
                decSum += FedCmsCommUtil.ToDecimal(pRow.Cells(intPos).Value)
            End If
        Next pRow

        Return decSum
    End Function
End Class