Could you give an outline of an example scenarios? For e.g. "print a line with a highlighted word in it" (alternative to print 'This is \e[7mhighlighted\e[m text\n'), and "print some text, move cursor to the second last word, delete it, move to the new end":

    print 'This is a line of text which is being line-edited in an imaginary editor.'
    if no line-wrap happened:
        # move left a manually calculated amount of 17 (wcswidth?), delete
        # everything to the right, reprint the rest of the line
        print '\e[17D\e[Keditor.'
    else:
        # You need to know how much fit on the previous line, and the
        # terminal's width. Let's assume 80, and that wrap happened
        # after 'imagi', so 'nary' was on the next line. Move up 1, move
        # right 63 (or do \r and move right 75), clear the rest of the
        # screen, reprint
        print '\e[A\e[63C\e[Jeditor.'

One of the problems with secondary channels is their synchronization with the main one: say you printed some text to the main fd, requested cursor position from the control fd, and then immediately printed some more text to the main fd — what cursor position will be reported?