I am not sure that a grill could make these 100% smoked spares any better. I used the grill for ribs before I learned how to smoke, but since I have had my smoker going the grill has hardly beeen touched, its been at least a year!
I don't want to be bothered with firing up the grill (it's a charcoal one) when I'm smoking ribs (or anything else really) just to finish something off for a couple of minutes. If I already had it going for some reason then I probably would because I'm sure it would taste good to set the sauce that way as ronp has mentioned.
Now in years past I have grilled spares on the grill and it's one of the reasons why I now smoke such things now. For one thing they are alot more tender and much tastier too. As far as the oven No never not gonna happen. I will always start and finish them in the smoker.!!!!
I would throw them on the grill if I were to cook them a day ahead or if they had to be placed in foil for several hours and they were getting cold. Other than than I just pull them off the smoker and foil them a while and eat.