Cateye Rapid 3 has a very bright flash mode and very dim one. Its brighter than the Mars 4.0 or Superflash and smaller too. Regulated output so will keep constant brightness for the full runtime (Mars and superflash are not regulated). The dim flash is very directional too so if correctly mounted wont be in the eyes of anyone drafting you.
Single AA and seatstay mounting possible.
http://www.cateye.com/en/products/detail/TL-LD630-R/