To follow up I now have this working and can insert flash movies .flv into the editor and play them with a flv player I have.

 

I didn't use the do_insert code, but instead modified the insertflash.js file to put flashplayer.swf?file= before the filename in the movie value= and the embed src= code

 

As I've hijacked the swf icon and dialog to do this the next task is to make both a swf and a flv icon/dialog