I know this post is a little old but I faced a similar problem and so thought I would post my solution here.
[Aside: I wanted to serve regular html/js/css via rook in conjuction with json responses to ajax queries with statistical solution - hence wanting to use R]
R.server <- Rhttpd$new(); # Create server
# Use a Builder to add a
staticApp <- Builder$new(
Static$new(
urls=c('/www/css',
'/www/js/libs',
'/www/js',
'/www/img',
'/www'),
root=getwd()
))
R.server$add(app=staticApp, name="static")
R.server$start()
Essentially my working directory contained contains a folder called www which contains all my static resources (in subfolders css, js, etc). In particular if the folder www contains a file index.html then this can be accessed via localhost:23702/custom/static/www/index.html
Other apps for more R-focused operations can be easily included in the builders construction.
Not probably would be cleaner with www moved to root = file.path(getwd(),'www') and a recursive search for all subfolders.
Hope this helps!