I remember reading something about this about 10 years (or more) ago, not sure where or even if it works, and this may not even be the article I was thinking about. This is a link to one of the saved pages for Z3's I have http://www.waycoolinc.com/z3/bimmerdump/65945.html
It does depend on the wiper module. You have to have the module for cars with headlight washers to have the programmable wipe feature (no idea why the feature is linked to headlight washers!). I have the correct module at home, waiting to be installed so I can tell you what the part number is - mine was bought second hand on ebay.