Assuming you don't mind having an external CAB file, it's not too hard… here's a (simplified) script that I'm using to permit end-users to customize a configuration file prior to deployment. The example code modifies the MSI (which is fine for testing), however our production code creates a copy, modifies it, generates an MST against the original and finally removes the copy.

This code doesn't actually “replace” the original file, but adds a second copy which gets installed instead.

Const MSI_SOURCE = “application.msi”
Const FILE_REPLACE = “config.xml”

Dim filesys, installer, database, view
Dim objFile, size, result, objCab

Set filesys=CreateObject(“Scripting.FileSystemObject”)
Set installer = CreateObject(“WindowsInstaller.Installer”)
Set database = installer.OpenDatabase (MSI_SOURCE, 1)

Set objFile = filesys.GetFile(FILE_REPLACE) size = objFile.Size

objCab.CreateCab “config.cab”, False, False, False objCab.AddFile FILE_REPLACE, filesys.GetFileName(FILE_REPLACE) objCab.CloseCab

Set view = database.OpenView (“SELECT LastSequence FROM Media WHERE DiskId = 1″) view.Execute

Set result = view.Fetch
seq = result.StringData(1) + 1 ' Sequence for new configuration file

Set view = database.OpenView (“INSERT INTO Media (DiskId, LastSequence, Cabinet) VALUES ('2', '” & seq & “', 'config.cab')”) view.Execute

Set view = database.OpenView (“UPDATE File SET FileSize = ” & size & “, Sequence = ” & seq & ” WHERE File = '” & LCase(FILE_REPLACE) & “'”) view.Execute

“This is possible using msidb which is part of the Windows Installer Development Tools.

I think you will need to do the following:
- extract cab file using “msidb -x”
- replace the file in the cabinet
- delete the cab file from the msi using “msidb -k”
- add the changed cabinet back using “msidb -a”

If you do not know the name of the stream you can use a tool like InstEdit to find out.”