Well, since that idea has occurred to you too, I guess it isn`t too dumb. But since you`re building it yourself and nobody else chimed in with a premade, it looks like there aren`t any commercial options for it.
Did you mean to write that you plan to use dyno HEADLIGHT at night and a super tail light in the day? Are you going with a red diode or white with colored lens? I never turn off my dyno lights either, but I kind of like the idea of more light in back for the daytime- currently have a plain ol` Seculite back there day and night.