Is this use of JavaScript prototype functions for attaching event listeners going to meet design goals?
Looks fine to me; but I think the this.evnt &amp; this.funct abstractions are unnecessary. Would always promote readable properties over shorthand (funct -> function etc), and I think that you should change the addEvent method to accept parameters. You can then clean up the code like so:

var CustomEditor = function(id) {
    'use strict';
    this.id = id;
    this.init();
};

CustomEditor.prototype.addEvent = function(event, callback) {
    'use strict';
    var element = document.getElementById(this.id);
    // old IE work around
    if (element.attachEvent) {
        return element.attachEvent('on' + event, callback);
    }
    return element.addEventListener(event, callback, false);
};

CustomEditor.prototype.init = function() {
    'use strict';
    this.addEvent('blur', this.onCustomBlur);
    this.addEvent('keypress', this.onCustomKeyPress);
    this.addEvent('focus', this.onCustomFocus);
};


See the fiddle.