That's just a work-around, but it does what you want and will work in calibre 3.xx
Create a regex-function (search box, pulldown "regex-fucntion" button "create") that does this:
Code:
def replace(match, number, file_name, metadata, dictionaries, data, functions, *args, **kwargs):
return '<span class"red">{}</span>'.format(match.group(0))
Then, with this regex-function active in your search box, you can select a text, then hit Ctrl-F Ctrl-R, it will do the job (or, same thing, click Search then Replace).