Please... nix the Splenda.
Since the FDA has no definition for “natural,” Splenda (made from "natural sugar" is the trade name for sucralose, a synthetic compound stumbled upon in 1976 by scientists in Britain seeking a new pesticide formulation.
It is true that the Splenda molecule is comprised of sucrose (sugar) — except that three of the hydroxyl groups in the molecule have been replaced by three chlorine atoms.
It is also scary to know that no study has been made of this substance further out than using it for 6 months or less. This means no one knows what it will do for a year, or 10.
Subjecting it to smoking temps is especially problem some and superflous since you are only sweetening the rub and the splenda does not behave the same way as sugar.
Lastly... there is no reason at all to use it if you have sugar problems so why risk it?