I use a Racktime TourIt rack and it has holes on the rear set at 50 mm apart. This works perfectly with Planet Bike's Taillight Rack Bracket http://ecom1.planetbike.com/3108.html .

This sets my Blinky 7 or Superflash at the rearmost part of the rack. When my Ortlieb classic roller bags aren't on, visibility is excellent 180 degrees around with the Blinky 7. With the bags on, the light is only visible from the rear (I use a Planet Bike Superflash then).

All Racktime racks have these mounting holes and I'd bet that all Planet Bike racks have 'em, or it has the hole for the vertical bracket that Planet Bike sells. Don't know about other manufacturers, but you might want to check this out for your rack.