I believe it all comes down to what one wants from the export/in Excel.
The XML of the TaskTree is relatively easy to decipher and analyse as the structure is quite clear.
Using VBA (Excel/MS Office programming language) you could import the MLO XML file and do stuff to the data.
Alternatively VBA could load the file to memory and traverse it there.
Again - one has to be quite specific on what one wants, like "I want all open tasks in Col 1, CloseDate (if any ) in Col 2 etc. etc.
If it is for cleaning up stuff (e.g. remove all completed tasks) and preparing the TaskTree for import again, that should be possible as well.
As I said - the MLO XML is well structured, so it is "just" a question of how much time one wants to put into the programming. :-)