It would be plain text, but with formatting commands such as HTML tags or LaTeX commands.
It wouldn't be rendered independently of the Rmd document into which it is inserted.
I should say, I am just evaluating R and RStudio to see if it is something we should migrate to.
Currently I am just speculating about potential applications rather than having a real live problem to solve, but this sort of automated or parameterised report generation would make it very interesting.