As of CR XI, the only way I know is to replace the export dialog with your own. You can add your own button to the CrystalReportViewer control and hide their export button.
Here's vb.net code to replace the export button with your own button/eventhandler...
Public Shared Sub SetCustomExportHandler(ByVal crv As CrystalDecisions.Windows.Forms.CrystalReportViewer, ByVal export_click_handler As EventHandler)
For Each ctrl As Control In crv.Controls
'find the toolstrip
If TypeOf ctrl Is ToolStrip Then
Dim ts As ToolStrip = DirectCast(ctrl, ToolStrip)
For Each tsi As ToolStripItem In ts.Items
'find the export button by it's image index
If TypeOf tsi Is ToolStripButton AndAlso tsi.ImageIndex = 8 Then
'CRV export button
Dim crXb As ToolStripButton = DirectCast(tsi, ToolStripButton)
'clone the looks of the export button
Dim tsb As New ToolStripButton
With tsb
.Size = crXb.Size
.Padding = crXb.Padding
.Margin = crXb.Margin
.TextImageRelation = crXb.TextImageRelation
.Text = crXb.Text
.ToolTipText = crXb.ToolTipText
.ImageScaling = crXb.ImageScaling
.ImageAlign = crXb.ImageAlign
.ImageIndex = crXb.ImageIndex
End With
'insert custom button in it's place
ts.Items.Insert(0, tsb)
AddHandler tsb.Click, export_click_handler
Exit For
End If
Next
Exit For
End If
Next
'hide the default export button
crv.ShowExportButton = False
End Sub
Then in the click handler you'd show a customized SaveFileDialog and eventually call the ReportDocument.ExportToDisk method. This way you can set the dialog's title and filename to something useful and of course set the default export type.