How to keep sauce spices in suspension
The easiest solution would be to add a thickening agent to your sauce.  Which agent will depend on how much you are willing to have the texture of the sauce change in order to achieve a good suspension.  One of the easiest methods, if you are also willing to alter the flavor profile in the direction of South Carolina or Germany, would be to add mustard.  Natural emulsifiers in the mustard will help keep the butter from separating and also provide a bit of thickness to keep the spices in suspension.  If you don't want to change the flavor profile, many recipes intended for the home kitchen suggest thickening the sauce with cornstarch.  Most commercial sauces will use a combination of modified food starch (which isn't readily available to the home cook) and xanthan gum (which is becoming increasingly available at most mega-marts), the usage of which has been discussed on this site previously [here and here].