How to onclick-trigger a non-native jQuery plugin's action?
This works.

Adding a targetable class to each slide, so that markup's:

    ...
    &lt;div id="myImg"&gt;
        &lt;a&gt;&lt;img src="myimg.png" /&gt;&lt;/a&gt;
    &lt;/div&gt;
    &lt;div id="accordion1"&gt;
    &lt;dt class="slide_1" &gt;
         ...
    &lt;dt class="slide_2" &gt;
         ...
    &lt;/div&gt;
    ...


then, in the init,

    ( function ($) {
        Drupal.behaviors.myEasyAccordion = {
            attach: function(context,settings) {
                $('#accordion1',context).easyAccordion({
                    autoStart:     false,
                    ...
                });
                $('#myImg',context).click(function() {
                    $("dt.slide_1").activateSlide();
                });
            }
        };
    })(jQuery);


Now, a click on the img in the #myImg div opens the target slide ... here, 'slide_1', as intended.