oh dear, what a silly mistake.
So I can confirm that I have to define the js in base.html
as I did in my question.
<script type="text/javascript" src="{{ STATIC_URL }}tiny_mce/tiny_mce.js"></script>
However this has to be in the header and not the body. Header is initialized first and hence there won't be any longer a ReferenceError: tinyMCE is not defined
Hope it helps someone else.