Custom Rmarkdown syntax highlighting in RStudio

I would like to add syntax higlighting for custom markdown in RStudio. Let's say I want text surrounded by <-- --> to be assigned one of the styles in the current rstheme (similarly to how, in an Rmd file opened in RStudio, text between * * gets the style for the .ace_constant.ace_language class). Any suggestions on how to do this? Many thanks!