So, the writing's coming along pretty well, and I ended up going with an explanation that werewolves have overactive immune systems to compliment their regeneration capabilities. So while they can eat a lot of the things that dogs and humans can't without long-term repercussions, there are a lot fewer things that they can eat without regretting it later (and unfortunately, Pepto-Bismol and Imodium A.D. are two of those things). No amount of chocolate can kill a werewolf, but that's mostly because it won't stay in the werewolf's system long enough before he pukes.
The werewolves in the setting I'm working with do it mostly based on trial and error -- they can't afford to make a list (or anything written concerning werewolves) of what can and cannot be consumed without regret, so they just try new things every now and then and make mental notes of everything they regretted later. The general pattern that seems to be forming is that the less effect something has on the body's chemistry (for better or worse), the better. Drugs and alcohol, for example, are just as likely to make a werewolf violently ill as medicine or a multivitamin.
So if the werewolves I've written the most about have any pattern of eating, it's that they tend to eat things that are basic and bland more than anything (especially the recently-turned), and whenever they do go on a culinary adventure, they do it at home where they can puke their guts out, crap their brains out and cry their eyes out all the privacy of their own homes.