Sugar substituetes will suck in que mop or rub. This applies to diet drinks, too.
Use cider - plain is fine - for a spray if you just have to spray. You can use it to moisten the meat to hold your rub, too.
Just drop the sugar in your rub along with some of the salt (they work together to develop the flavors).
If you feel like you must have some sweetness I'd recommend using turbinado sugar in place of the brown or white sugar and perhaps put it on the meat before you put your rub on.
Southern Brethren BBQ Competition Team
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KingFisher Gator Rotisserie cooker, WSM, Stainless 5 burner with IR gas grill, Turkey Fryer, Weber JD Commemorative grill, slightly used Bandera chamber for future use as a cold smoker, Masterbuilt 40" insulated ELECTRIC smoker