The only way I hold anything in the Nesco is if it has bottom and side heating. If it's side-heat only (for roasting) then it won't be worth a hoot on keeping the food heated through on bottom and in the center...been there, done that.
There shouldn't be a problem with the parboil, smoke and grill-sear tonight.
If you have a larger crock-pot would work great for this purpose. You will need to do temp checks with a probe to be sure. Not sure how mealy the texture will get after a long hold like that, but it can be done. Keep the cooker temp to around 140* as measured from near the center of the food in the cooker...you wouldn't need to puncture the brats, just get in between them and contacting the food...this should tell you what temp the cooker is holding at. Minimum recomended is 135*, but there's no room for error with thermometer readings at that temp. Also, the food must be fully cooked to 165* then go straight to the holding appliance, or reheated to 165* (treated as leftovers), then held @ 140*.
Eric