Yes, I have. It is actually really simple to write a defensive publication.
You can see various examples of how defensive publications are written at ip.com. Basically it comes down to this: one or two pages, and at least one diagram to describe interaction between components, dataflow, and so on. It does not take that long to write it (about an hour or less for people with some experience).
Very soon Open Invention Network will travel to several conferences (Akademy, GUADEC, COSCUP are planned) to provide hands on assistance with writing defensive publications.