WiX: Make component upgradable, but not uninstallable - wix

Question

I am creating an installer using WiX (and I must say I really don't enjoy it, but that is less WiX's fault and more MSI's. And I absolutely applaud Rob Mensching's helpfulness here and elsewhere).
Is it possible to create a component such that it:
can be updated (via patches and major upgrades)
Files that have changed since install will not be uninstalled (it doesn't matter whether they're also not uninstalled if unchanged)
Ordinary file versioning rules apply when updating (that is, unversioned files that changed since install aren't updated, others are)
I am beginning to think this isn't possible. Marking a component as permanent means the component won't ever be uninstalled, which means it isn't updatable (at least not via major upgrade; how about patches?).
Marking a component NeverOverwrite won't prevent it from being uninstalled, even if the user has changed the unversioned keyFile (thus the user's work is deleted).
I appreaciate all definitive answers, even if they are negative.

Answer 1

I did not try it, but you probably can use a custom action to do this. See From MSI to WiX, Part 5 for an example. The Custom Action Type 38 example includes this:
<Directory Id="INSTALLDIR" Name="Minimal" LongName="MinimalInstallation">
<Component Id="Component1"
Guid="{A77C5B06-132D-4884-8E17-EA10A83C812D}">
<File Id="ReadMe" DiskId="1" Name="Readme.txt" Source="Readme.txt" Vital="yes" KeyPath="yes" />
<!--RemoveFile Id="RemoveCopy" Name="ReadMe2.txt" On="uninstall" /-->
</Component>
</Directory>
....
<CustomAction Id="CopyReadMe"
Script="vbscript">
<![CDATA[
Set fso = CreateObject("Scripting.FileSystemObject")
path = Session.Property("INSTALLDIR")
fso.CopyFile path & "ReadMe.txt", path & "ReadMe2.txt"
]]>
</CustomAction>
<InstallExecuteSequence>
<Custom Action="CopyReadMe" After="InstallFinalize">Not Installed</Custom>
</InstallExecuteSequence>
Here Readme.txt is installed, and using a vbscript custom action copied to ReadMe2.txt. If you do some version and/or modification date checking in vbscript, you can control yourself whether the ReadMe2.txt is overwritten or not. And without the RemoveFile, wix will not uninstall ReadMe2.txt, because it does not know the file exists.
For an upgrade it might be possibe to use a different custom action to copy ReadMe2.txt back to Readme.txt before installing. That way you can let wix handle the Ordinary file versioning rules to determine if Readme.txt can be updated. Then you don't have to add version handling to the vbscript.