fazerbox's trackmap has it. Also on the dashboard of the formula ISI the two lights on either side of the rev lights are green when DRS is on and yellow when it is not activated.
View attachment 6689 TrackMap plugin. http://isiforums.net/f/showthread.php/3609-TrackMap-0-9beta-Plugin-for-rFactor2 In the same kind rfdynhud.