This option should contain a function name to be executed each time a editor instance is initialized. The format of this function is: initInstance(inst) where inst is the editor instance object reference.
Example of usage of the init_instance_callback option:
function myCustomInitInstance(inst) {
alert("Editor: " + inst.editorId + " is now initialized.");
}
tinyMCE.init({
...
init_instance_callback : "myCustomInitInstance"
});