It should be much easier with the excerpt field because it doesn’t use a WYSIWYG editor. Something like this (also on JSFiddle):

jQuery(document).ready(function ($) {
    // Set your character limit
    var count_limit = 10;

    // Set the initial symbol count on page load
    $('#excerpt-wc span').text($('#excerpt').val().length);

    $('#excerpt').on('keyup', function () {
        var char_count = $(this).val().length;
        var excerpt = $(this).val();

        // Update the character count on every key up
        $('#excerpt-wc span').text(char_count);

        if (char_count >= count_limit) {
            $('#excerpt-wc').css('color', 'red');
            $(this).val(excerpt.substr(0, count_limit));
        } else {
            $('#excerpt-wc').css('color', null);
        }

    }).after('<p id="excerpt-wc">Count: <span>0</span></p>');
});