I haven't got any images of what I plan to do but I remember when @coldharbour did that unicorn TT bike he chopped a bit of the seattube out and brazed a 1mm is steel sheet on instead. I think it will be a simple solution if the seat tube is otherwise in good condition.
I don't mind - it's not my bike.
I just thought that the method could be used to mend the broken seat tube.