So far, this whole thread is then more or less useless, considering we only got tricks or loopholes. I agree that looking up at the laws itself is the answer. That's unfortunate it seems quite difficult to get those rules thru the net, though. Do you have a link?
For me, I would just take a bottle down to the border, and ask if there are any restrictions on the casual importation of this particular item. But I live 13 blocks from a border crossing!
Commercial importers hire Customs Brokers in order to provide these sort of services.
The result you want is something like an official identifying the particular document that restricts chemical importation, and then taking you through that document to identify whether those restrictions apply to the product you seek to import - making sure to identify which part of the document applies to that item.
Unfortunately, all of the systems designed to deal with these sorts of things are designed to deal with one of two general scenarios:
1) complex and well detailed but voluminous rules that apply to commercial importations. Accessing those rules and making sense of them is a fairly specialized endeavour; or
2) much simpler but also much less comprehensive rules that are designed to be used by individual travelers and made relatively easy to access by the general public. Unfortunately, those general public rules are unlikely to deal with Rodinal.
Assuming that you are buying a small amount - something like a couple of bottles - I would recommend you just declare it when you arrive at the border, and if there are any questions, show them this, with the note indicating that Adorama ships it to Canada, as well as the fact that it is a film developer: