Creating reports in R
In conjunction with the tools that you have discussed (rstudio and sweave/knitr) you might want to look at the pandoc program which will convert from the formats created by those tools into other tools including word .docx files (and others). So one option would be to create a basic template file in R or Rstudio and use the built in tools to work with and view it while creating it. Then when you are ready to share the document (with embedded graphs, output, etc.) you can convert to a .docx file and colaboraters can comment and track changes.
Other options would be to use R2wd (as mentioned in the question, but note that it uses non-free tools) or R2HTML or rtf packages to create those types of files which can be read in using ms word as well.
Have a look at Sweave (in base R) and the knitr ---and everything in the CRAN Task View on Reproducible Research which focusses on the very same topic.
Maybe you can import the knitr'ed HTML into Google Docs. There you have not only track changes but real time editing. I did not try this, though.