The official word on catsup can be found here
: "There are many Barbeque Sauces and Catsups available that are compatible with the McDougall diet. Please check labels carefully and choose those that are low in sugar & salt and avoid any with added oil."
Bear in mind, however, that these things are supposed to be used as condiments -- if you like to slather huge amounts of them on your food, as I sometimes do, the sugar remains a concern, where it might not be if you were just spreading a tiny bit on something.
Also, "available" is a relative term depending on your location. I haven't found a single catsup or barbeque sauce that's low in sugar -- though I think they are available in health food stores at a large price.
My advice -- if all you've got is the standard Hunts/corn syrup brands -- would be to use them very sparingly, if at all. A teaspoon on your veggies, not a half-cup. Once you can find some low-sugar varieties, then it's just a vegetable sauce you can use freely.
The advanced advice -- not something I'd suggest when you're still just starting out and have lots of different stuff to think about -- is to make your own catsup. Just google for "catsup recipes" until you find one you like, then modify it by taking out all the oil (if any) and most of the sugar. I saw Jamie Oliver make a nice catsup on his show, so I Googled "Jamie Oliver catsup recipe" and used that for my starting point. Left out the oil, left out the two cups of brown sugar, used a bit of apple juice to saute the vegetable ingredients, and added one tablespoon of molasses at the end. It was delicious and as sweet as I would want.