I recently wrote a script for an RPG-in-the-making that attaches extra elements (in the form of another GUI) to the existing PlayGUI using PlayGUI.add().
Basically, I made a GUI (called SRPG_HUD) containing the extra elements I wanted to attach to the PlayGUI, then used a script to add it in.
package srpgHUD
{
function playGUI::onWake(%this)
{
if($connectedToSRPG)
{
PlayGUI.add(SRPG_HUD);
}
parent::onWake(%this);
}
};
activatePackage(srpgHUD);
The client would need an Add-On with client scripts/GUIs to be able to do this, though.