I have a WiX installer that includes this declaration of directories:
Did you try using custom actions?
You can use one of these custom action to change the property value during install:
For example:
<CustomAction Id="ChangeDir" Directory="INSTALLFOLDER" Value="[SomeValueorPropertyhere]"/>
2.Schedule the action during the InstallExecution phase (must be after the CostFinalize step):
<Custom Action="ChangeDir" After="CostFinalize"></Custom>