To say this is hot off the presses and poorly tested is to flatter it but here's my first attempt.
https://github.com/psidnell/oo2taskpaper
I basically reverse engineered
Fletcher Penny's markdown plugin to work out that it was a an xsl template - about which I know little.
Download the zip:
https://github.com/psidnell/oo2taskp...ive/master.zip
and double click on the TaskPaper.ooxsl bundle and OO4 should install it.
Then you should see a "TaskPaper (text)" option when you export.
So far it can export a basic outline successfully but looking at the input document I have a suspicion that adding formatting could cause issues.
But it's a start...