I would always bring a back-up, even if it's a collection of fresh fruit, nut bars, chips (the catered meal would be indulgent, so why not be a little indulgent too?!), whatever you can manage depending on how you're traveling. I had a great experience recently at a wedding that I expected not to be able to eat anything at - I'd prepped a whole meal and was going to go out to the car to eat quickly at some point, but wound up being able to eat a lot of the many salads and beautiful fruit. One of the catering managers helped me with what was safe and what wasn't. Of course, it helped that it was really relaxed, buffet-style and everyone just grabbed plates when they were ready to eat (they didn't call up specific tables).
No need to involve the bride or groom, unless to get the caterer's info - they have enough going on and will likely stress over that additional detail, even if you tell them not to. Contacting the caterers in advance is a great option, as is offering to pay for any price difference. I don't think many places are likely to give a "discount" if you don't eat, since they're pricing by attendance and not by who is eating what - the caterer still has to provide drinks, etc., and it's all part of the contracted package, especially if the caterer is tied to the venue.
Remember, too, that with all of these online review sites you can help encourage "good behavior" by sensitive and helpful caterers by providing good reviews and by reviewing those with snide servers or those that essentially poison you with bad reviews. Lots of good websites - The Knot, Wedding Wire, and others have lots of quick and easy review options, and that really helps those of us planning weddings, too! Chances are, if that wait staff was rude to you, they were also rude to someone else.