In principal it won't be such a big deal to write such a parser. But the problems will appear in the details. One has to create a kind of meta syntax - processing statements - for all these R chunks. E.g.
@RCODE quiet@ a <- matrix(c(1,2,3,4,5,6), nrow=2 byrow=T) @/RCODE@
foo foo
@RCODE insertAsTable noCommandLine@ print(a*2) @/RCODE@
etc.
But this would be mean to reinvent the wheel twice in some sense. Markdown to LaText and Sweave for LaTeX works fine.
On the other hand I use quite often the R bundle command 'Execute Selection/Document in R and' > 'Insert Result' for such things.
Of course, to have such a pre-parser for expanding R code chunks and insert the result depending on the current language (HTML, LaTeX, Markdown, Plain Text) would be nice.
--Hans