Probably an easier way to do it, but in case it is of use to anyone else:
This lets you highlight a specific character, when all you know is the position of the character in the source, not what line and character number that is converted into by the editor.
var highlightedText;
function highlightParam(pos){
if (typeof highlightedText === "object"){
highlightedText.clear();
}
var ttl = 0;
var line = 0;
$('.CodeMirror-line').each( function(){
var s = (ttl === 0) ? ttl : ttl + 1;
var l = $(this).text().length;
ttl += l;
if (line > 0){
ttl++;
}
if (pos >= s && pos <= ttl){
var diff = ttl - l;
var posAdjA = pos - diff;
var posAdjB = posAdjA + 1;
highlightedText = editor.markText({line: line, ch: posAdjA}, {line: line, ch: posAdjB}, {css: "background:yellow;font-weight:bold;"});
editor.setCursor({line: line, ch: posAdjA});
}
line++;
} );
}