<?xml version="1.0"?>
<layout>
    <default>
        <reference name="head">
            <action method="removeItem">
                <type>js</type>
                <name>mage/adminhtml/wysiwyg/tiny_mce/setup.js</name>
            </action>
            <action method="addItem">
                <type>skin_js</type>
                <name>js/path/to/your/tiny_mce_setup.min.js</name>
                <params/>
                <if/>
                <condition>can_load_tiny_mce</condition>
            </action>
        </reference>
    </default>
</layout>