Excel Pivot filtering on calculated fields

You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:

ExcelLibrary

This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.

It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.

ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.

You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.

There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.

Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)

Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License

Here some example code for ExcelLibrary:

Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:

//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");

//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;

//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();

//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();

adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();

//Add the table to the data set
ds.Tables.Add(dt);

//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);

Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.

Create a procedure to change all the page fields. This one loops through all the worksheets in the workbook and if the worksheet has a pivot table, it changes the page field to match the page field on the pivot table passed to the sub.

Sub ChangePage(pt As PivotTable)

    Dim strPageValue As String
    Dim wks As Worksheet
    Dim ptUpdate As PivotTable

    Application.ScreenUpdating = False
    Application.EnableEvents = False

    strPageValue = pt.PivotFields("Area").CurrentPage

    For Each wks In ThisWorkbook.Worksheets

        On Error Resume Next
        Set ptUpdate = wks.PivotTables(1)

        If Err.Number = 0 Then
            On Error GoTo 0
            wks.PivotTables(1).PivotFields("Area").ClearAllFilters
            wks.PivotTables(1).PivotFields("Area").CurrentPage = strPageValue
        End If
        Err.Clear

    Next wks

    Application.ScreenUpdating = True
    Application.EnableEvents = True

End Sub

Then place a call to this procedure in the worksheet code itself and pass the pivot table you changed:

Private Sub Worksheet_PivotTableUpdate(ByVal Target As PivotTable)

    Call ChangePage(Target)

End Sub

Change the name of the pivot field from "Area" to the page field you need, for example "US_Region".

This may not be suitable if people are frequently changing the structure of your pivot tables in other ways, as it will trigger each time the pivot table is changed, including when it is refreshed. It will also give an error if users remove the specified page field.

If the pivot tables are static though, this might do.