Have you tried creating a custom script which pastes in the tag. You can use the following to paste custom tags into the designer.
document.getElementById(
'CE_Editor1_ID').ExecCommand(
'PasteHTML',false,'<flashvideo src="wiggle.flv"></flashvideo>');
Furthermore, if you get a HTMLObjectElement (el_tag below) from inside the designer then you could use
CuteEditor_GetEditor(el_tag).ExecCommand(
'PasteHTML',false,'<flashvideo src="wiggle.flv"></flashvideo>');
Hope this helps. Mark