Bill - Welcome to the club that nobody wants to join. I substitute Splenda for sugar in any recipes. Don't use anything else, as it won't hold up to higher temperatures. To attempt to duplicate a brown sugar, consider using a Splenda/black strap molasses combination. (don't need much molasses). And keep in mind that overall, the amount of rub/sauce is still pretty minimal.
As you probably know, you can purchase low-carb (low sugar) Ketchup, albeit it is a bit expensive. I try to stay true to unprocessed foods, and make most condiments myself.
Good luck.