Is the an equivalent to Onyx for diagramatically producing R code for data preparation, such as recoding data, creating new variables, matching/merging datasets, etc?
Not to my knowledge. Making an R "LabVIEW" for data processing is an interesting idea. But I don't think anyone's done it as yet.
http://www.rstudio.com/shiny/ is maybe a move in that direction, but not what you desire.
Go for it!