Post XML form using httr

I am trying to POST an xml object using httr

In my application, I am creating the XML object like this:

  doc <- htmlTreeParse(conts, useInternalNodes = T)
  xmlFormula <- xpathSApply(doc, "//*/textarea", xmlValue)
  x <- xmlParseString(xmlFormula)

Then I modify the xml on the fly

and now want to post it using httr without saving it to disk and uploading from a file

  POST(MYURL, body=x) # this won't work 

Is there a method to make the xml object httr friendly?