The simplest circuit would be a pulser connected to a flip-flop driving a pair of MOSFETs powering the headlights. This idea I am trying before I can upload the schematic, some of my 2B1 headlights work on high beam only and are suitable for this circuit. My circuit will use a 555, 4013 or 4027, and IRF630/ 640 MOSFETs.
If you really want to get their attention you need high intensity quick flashes that normal vehicle lamps are not capable of providing.
I can't find the site I was looking for but did find this one:
**broken link removed**
I was actually looking for one about the instant LED turn on in motorcycle brake lights making them safer by giving the driver behind them an extra 150mS to stop.