I just called Bulk Barn and Rainbow Foods to ask about their policies on customers bringing their own containers. Rainbow Foods is all for it, and they have no problem. Bulk Barn however is not so considerate. They apparently have too many allergy concerns, which I guess is fair. But they also don't have a scale for measuring containers, the scales there are pre-programmed for their own containers alone.
Guess I won't be going to Bulk Barn anymore! At least not unless I need dog poop bags.
At least I know now.
I also found out that Ottawa also has a few Natural Food Pantry s. They aren't super close, but they do have bulk bins with much more variety than Rainbow Foods, and they allow you to bring your own containers etc for use. So that's awesome!