Yeah, bleach will be no problem, esp since you won't have plants in it for a while. Personally, I don't think your cab needs to be "germ-free," just clean. I usually use rubbing alcohol to clean between grows, and never had any problems with mold or mildew.
I used Ona right behind my exhaust fans to kill my skunky mj odor, but the cab was located in a closed garage, so basically there was always some concentration of Ona was in the air in the garage, and that air was flowing through the cab for most of the flowering period. I haven't noticed any effect on flavor or potency of the buds I got when using Ona compared to when using nothing at all to control odors, so my conclusion thus far is that Ona is perfectly fine even in the grow room in reasonable concentrations. However, it's hard to say what kind of concentrations you'll get with a bucket near you intake vent, I would think near exhaust or at least away from the intake until you can try it out and see how potent the Ona smell is. I believe that Ona is comprised of a bunch of different natural aromatic oils (and probably some other stuff as well), so too much of this "oily" vapor in the air could certainly end up affecting your buds, but as I said, if it's just enough to mask the mj odor then your plants should be fine with it. It still would probly be best to locate it not too close to the intake, but I wouldn't worry too much about that right now if there's no other choice, since it seems to be pretty neutral in the end.