Technical writers would most likely mostly benefit from understanding the software through tests that are easy to read, i.e., are written in natural language. In addition, having access to some concrete examples would further help their documentation’s accuracy.
It so happens that these properties map exactly to the kind of tests that would be produced by a team doing BDD, ATDD, or specification by example. If one of the goals of the team were to produce accurate technical documentation, then it becomes natural to invite the technical writers to the specification workshops or planning meetings, or any other meetings during which scenarios are written and discussed, and examples are being created.
Look at Concordion, RSpec, Cucumber, and FitNesse on the tools page.