Nope - iFrame didn't work either.

Working on the same general idea, I tried defining an additional <div> in the openHtml, then used a ClientAction call to a new function, passing the ClientID, to fiddle with its innerHTML:
Code:
loadPayPalScript : function(clientID) {
    var scrDiv = document.getElementById('paypal-script-container');
    var script = "<script src='https://www.paypal.com/sdk/js?client-id=" + clientID + "'></script>";
    scrDiv.innerHTML = script;
},
But that too failed to work (the JavaScript worked OK - I can see it all happen as expected in the browser's debugger - but the script still didn't get loaded ).

Mike