How to set a button's active state in WYSIWG (Redactor plugin)?
I've done this by adding my button to activeButtons and adding my inline tag to activeButtonsState after adding button in init function. 

In below example, cite is the tag and citebtn is the button's name.

init: function()
{
    ...
    if($.inArray("citebtn", this.opts.activeButtons) &lt; 0) {
        this.opts.activeButtons.push("citebtn");
        this.opts.activeButtonsStates.cite = "citebtn";
}