Is there any way to get the Custom button enabled for MPEG1 renders? Seems as though I used to have it enabled and could change bitrate, etc., but maybe I'm just crazy. Right now, it is disabled on MPEG1 and MPEG2.
Try Make Movie / Write your movie to file... / Select MPEG-1 from Format and click "Advanced Render" / specify a location and filename and select type MPEG-1 / click "Custom"