Thanks. This would be very nice.
p.s.command-line is not my favourite. Fritzing is simple to me.
May be simple GUI tool where you can import several Gerber projects and with drag and drop to a single Gerber project.
Maybe someone know's a good software?
But best is of course in a future version making a v-cut in Fritzing a simple as placing a trace. 
BTW seeed fusion is the place where i order my pcb's. They accept v-cut, they do not mention any parameters.
Is it possible (and easy) to do it with a text editor in the gerber file itself? May someone could place a example.