I just solved my own problem !
The following line from RTE_javascript.asp generates the prompt regarding insecure documents:
textArea.parentNode.insertBefore(iframe, textArea);
All you have to do is make thye SRC property of the iFrame KNOWN :>
Set it to some other file in your HTTPS folder like this...
iframe.setAttribute('src', 'loading.htm');
After assigning the valid filename (which no doubt carries the https flag with it), you will find that the offensive 'insertBefore' command will now function!
dBee