In XEmacs there is nice "subwindow" glyph type, which could be used to
display contents of foreign application (such as video player).  Internally
it is just ordinary subwindow of fixed geometry with API to get its XID,
which you specify to foreign application to render to.

Is it possible to do the same in GNU Emacs or would it be hard to implement?

Unfortunately xwidget functionality is limited to "webkit" type of widget,
and it does not provide API to get XID of the widget's window.

Thanks

-- 
lg