If you have a rack, I'd say go with a B&M 4D Toplight. Takes 2xAA, no blink mode, but acts as both a reflector and a light. If it weren't for your prerequiste of strictly AA/AAA, I'd suggest the Hotshot and just program whatever output or flash mode you want. The new NR Solas supossedly has a 'group ride' mode, but it's USB powered as well.

I've got the 4D, and it does a good job of side visibility as well. I can stand at my handlebars and still see the light.